To support, a certain mindset is needed. This goes for all roles, but as a support you're more likely to get flamed and you always feel like you carry more weight on your shoulders than well, the carry.
I couldn't find the original source for the things below, so I'm summing them up here. I believe if you keep all of this in the back of your mind, it will make you a better player and a better teammate.
Be positive. Don't question other people's decisions, rather say what you think would be best.
E.g: "Luna, I think a BKB would be good for you here." is better than "Luna, why are you not getting a BKB?"
What can truly help is to warn your team of things that may happen or are happening, not things that have happened, those are for the replays.
Take a break after a loss
Switch between unranked and ranked if you get some losses in ranked
Play heroes with a big impact.
Communicate with your team. It increases teamwork and reduces flame.
It's all about confidence in your team and not being greedy yourself.
Mute the damned. There is no helping certain people, and fighting them will not help at all. Mute them the second you are certain they will have nothing of value to say in the match and forget they exist.
History is irrelevant. There's no point in talking about something that happened 10, 5 or even 2 minutes ago while the game is still in progress.
It is however relevant when watching replays.
NEVER point out what someone did wrong. There's very little chance they will take into account what you, a random pub, said, let alone apply it to that exact game.
"Because Rhasta died so many times, this Slark is now super fat." are some of the worst things you can say.
Instead you should be asking yourself what you and your team can do to stop him now. Can you outfarm him, can you gank him, how strong is he, etc.
"We lost". No you didn't. "ff plz" is something you should NEVER be saying. People that give up when they fall behind, will never learn to play from behind and thus will never be able to turn around a game.
A lot of pub players will start to get arrogant and overextend and/or start buying things like rapier when they are ahead, this is your chance to strike back.
Don't queue up if you can't focus on an entire game without giving up on it.
Don't play when you're tired.
Avoid passive farmers, especially junglers.
Personally, I think I'm good at leading my team in-game, even if they are 4 randoms. However, if there's someone else that also feels that way and he's disagreeing with you at every turn, just leave him be and only give crucial info.
The wards below are just examples. There are obviously many more options.
Safe lane ward to keep tabs on the enemy offlane. Allows you to safely pull and to know when to go in and harass/kill the enemy offlaner. Can also be put at the corner of lane, river and jungle or closer to your tower.
High ground mid (optionally also seeing top rune): To assist in pulling creep aggro and spotting early ganks.
Offlane camp blocks. Only block the hard camp if your offlaner does not want to pull this camp himself. Most often used in a 3v1 situation.
Offlane ward to spot enemy movement.
Rune/Rotation spotting wards.
Courier sniping wards.
solo support/with greedy 4
Try to help out 1/2 of your lanes as much as possible. Ward defensively and try to deward so your movement between lanes is as well hidden as possible.
You buy the lion share of support items, your fellow support can take over when you want to get a core item and his are up.
Make sure you use that little bit of early gold you have all the time.
If both you and your carry are low, give him your Healing Salve, go to base or sanctuary and walk back to lane.
Actions and movement
Don’t just randomly go around the woods before the horn because you might get screwed over by a possible aggro trilane.
Try your best not to pull at 1 Min game time as your carry usually does not have enough damage or experience to last hit under tower. Wait until the 3rd wave to pull and while waiting you can harass the enemy offlaner so that your carry can get some last hits and either get a Quelling Blade or his Boots of Speed!
If you are babysitting, focus on denying creeps. Note that you should only really be babysitting your carry when both you and your him can get killed by the enemy laners, you can't kill them unless they overcommit and your carry can't survive on his own.
Don't single pull the small camp unless you want to push the lane.
If you want to get a kill, know you can but don't see an opportunity yet, don't harass the enemy laner. Stay close and let him think he can safely lane.
Be very careful with pulling the lane if you have a 2v2 or 3v3 on your hands. Make sure your carry (or you) doesn't die. Abuse creep aggro to net your carry easier last hits instead of pulling.
If you can't allow your carry to get farm but he won't die either, abandon the lane and gank mid/safe lane.
Try not to draw creep aggro when harassing the enemy offlaner. It messes up the equilibrium.
When pulling, stand to the side of the enemies so you can see them coming before they disrupt your pull (be aware of your matchup though, a Clockwerk can easily kill you this way).
If you have 2 ranged creeps and no option to pull (because no camps, too risky for your carry, etc.), try to aggro the enemy creeps onto your ranged creeps.
Always check your opponents armor, regen, HP and damage before you start harassing. This way you know if you can just stand and right click them.
If you can't just right click harass to outlast them, use mana harass.
If you can't harass the enemy out of lane in any way, leave, because you're useless in that lane.
Against 0 heroes Stack and pull as much as possible. Maybe do a single pull with a siege creep to take down their T1 tower.
Against 1 hero
One of the supports should pull/stack, the other should be zoning out the enemy offlaner. If you can't zone him out on your own, both of you should focus on getting a kill and then pulling.
2v1 instead of 3v1
Try to keep the enemy hero from getting his key levels. When he gets there you want to rotate to another lane before he does or pull more.
Your carry can 1v1 but you can't help in killing the enemy offlaner
In this case you can rotate to the enemy safe lane or mid immediately.
Against dual lanes
There are several options here:
Your lane is stronger: They are stupid. Punish it.
Their lane is stronger: focus on protecting your carry at all costs.
3v2: do the same as in a 2v1.
You have a jungler: let him jungle until he is strong enough to kill the enemy dual lane with you.
Against an aggro tr[/h4]i lane
Let your carry farm if he can, there's no need to get kills.
If he can't farm the lane, swap your lanes or try to get kills on the enemy.
If the enemy lane is FAR better than yours, move to another lane or try to make them over extend into your tower.
Leave. The worst thing you can do as a support if you're in an unwinnable lane is stay there and soak up xp while doing nothing. Let him get solo xp so he's at least overleveled and underfarmed instead of underleveled and underfarmed. Oh, I should mention I'm assuming he knows how to play and doesn't over extend and die to get 2 creeps per minute and just sits back and soaks xp to get overleveled, you might have to mention that to him (in a polite manner) if he's bad.
Go to different lane and stay there to push forcing pressure off your safe lane (often called "making space")
Hate on mid again
Ward aggressively instead of defensively. People often think when they're behind they have to play defensive. However, warding the opponent jungle and running at their Anti-Mage while he's trying to farm can slow him down/get a Town Portal Scroll reaction and they waste time chasing you (again, making space)
You should have a level advantage on 2 of your cores from 2 solo lanes and you protecting mid, use it mid game to kill people and turn xp into gold
See who's doing well on your team, follow them around (if they need +1 to kill, if they don't let them do what they're doing and just sit far behind them in case of ganks) and help them kill people so you get assist xp/gold and they're safer because you can sacrifice yourself to make sure the person doing well doesn't die.
Hope your carry isn't full ****** and can make use of solo xp
Farm jungle yourself. It might actually be better your carry gets solo xp and you get jungle farm from pulls, but that's a judgement call.
Stack ancients/large camp. Use at your best judgement though, if your mid is getting pushed down at 5 minutes you're probably not going to get to farm those ancients. You can hang out while your carry kills it for some levels.
If you're a support who can jungle ( Crystal Maiden, Sand King) doing so will also make the other team forget about you, giving you higher chances of success with your next gank.
Basically, there's tons of things to do when you lose laning phase unless you get crushed and die 3 times each. Then the game might just be over (but again, it never is).
Don't stare at the cliff while you're warding. When warding, shift queue. Place the ward and Shift click somewhere else so the motion is fluid.
Want to make this worse for your opponent? Stare at a cliff you're NOT warding while you know they have vision of you.
If you feel you're getting yourself into a warding war, stop dropping Observer Wards there. Maybe you can drop another Sentry Ward but that's about it.
After you upgrade the Animal Courier, your next item should ALWAYS be a Town Portal Scroll.
Suddenly being on the other side of the map is a strong surprise and easy kills.
Even easier is TP'ing when someone is tower diving. Free double kill for you.
Even (especially) if you're a support, don't let last hits go to waste.
When pushing, try to be the first to kill the ranged creep
When pulling, last hit those neutrals
Play from the fog as much as possible. Doing this, you deny information to the opposing team. Seeing a support hero in lane makes the other lanes feel safe and comfortable. Support hero that's off the enemy radar is a threat anywhere on the map.
Fight the urge to Town Portal Scroll to a lane when you're in the fountain. Keep the cooldown fresh and save gold for when you're actually needed.
Assist gold is always evenly split among heroes in a 1300 radius of the kill, even if they didn't damage them.
When you narrowly survived a dive, mind your position, but don't run back yet.
If your team turns the fight you can maybe throw out another spell and in the worst case benefit from assist gold.
Ask yourself this one question all the time: "Am I safe?"
Always travel with someone else, even when warding.
Looking for some farm? Take farm close to where your team is or a tower of yours.
Are you going to iniate? Make sure your team is ready to follow up so you don't die.
Take opportunities for solo xp when you can get them. If your mid leaves the lane you can take the lane during their absence.
Some heroes like Crystal Maiden or Sand King can also make use of jungle on their own. Don't be too greedy doing this though and always have a Town Portal Scroll on you.
If your carry is farming alone or very safely, you can pull as much as you like to get the xp and gold from the jungle camps.
Initiation is ok if you are ganking WITH VISION. It's not ok when 4 enemies are off of the map and you see 1 in jungle. Always assume that every hero missing from the map is near your target. Otherwise, please let your offlane or mid initiate.
Everytime you want to cast a spell cast it at max range away from the teamfight.
Supports should always have 1 eye on the lane, 1 eye on the minimap and another eye on the Observer Ward CD.
A support without mana is often useless. Walk back to base or sanctuary to get mana.
As a support player, you will spend a lot of times walking around warding, trying to assist in kills or simply waiting in the fog for your trap to snap or to find a lonely hero to gank. During that time, try to think ahead to the team fights. Try to anticipate the enemy battle order, their items, their positioning.
After a pull is the best time to Smoke of Deceitgank. The enemy offlaner will report missing but tell his teammates you are pulling.
Try to always gank with your fellow support player unless he has 0 ganking potential or you have plenty. Just make sure you get the kill.
Later in the game if you want to Smoke of Deceit, deward the area where you're smoking first or make sure it's not warded.
Once Smoke of Deceit is used, there is a cloud travelling quite a distance and this can be seen by enemies. So avoid using it too close to a creep wave since the enemy might spot it.
Always try to secure the 4 min rune for your mid. It's the most important rune in the early game.
Obviously rune control is always important, not just at 4 min.
mid to late game, a DD rune can easily secure your team the victory. (As recently shown by 4dr on Clinkz, see below)
Rune control is normally left to the 5 position.
Check your opponents and your own positioning before you go the rune spot. Will you be there in time, can/will the enemy kill you?
If you can't contest the rune, try to at least make sure you know which one it was.
Check the opposing mid's hero/items to see if they're rushing Bottle. The movement from pulling a camp at ~1:42 to securing the runes at 2:00 then sniping the Animal Courier before 3:00 is just so nice.
Ganking the enemy mid is really easy during the first night. From the moment he gets into the river, you can basically kill him. Even when the enemy is under his tower mid, you can still easily gank him.
Just make sure you know the vision between the T1 and T2 tower mid.
Don't forget that your safe lane has a gank path from the side shop too. It's heavily underused and really effective. Especially when the enemy has an Observer Ward in your jungle to spot ganks.
Learn the ganking potential of your enemies. You have a mid Sniper vs that Storm Spirit? Hide at your T1 tower to counter initiate when he inevitably jumps your mid at lvl 6. Storm Spirit died/went back to base? Check the map for potential ganks on your allies and have your Town Portal Scroll ready. Offlaner about to hit 6? Protect your carry or kill him before he starts to rotate. He's missing? Have Town Portal Scroll ready and be careful.
Gank on even minutes after pulling (note that you will probably not be the only one doing this) OR make a move before the odd minutes and gank while on your way.
Get the kill or don't gank (unless it's the enemy safe lane)! The addition of shrines means a failed gank is just a net loss for you due to the resources you invested in rotating.
When to gank
Your carry has free farm and can easily control the opposing offlaner OR you can't control the offlaner along with your carry anyway.
Your offlaner got zoned out hard, but he still has kill potential if you rotate over
Your mid-laner is having a rough time
Your opponents are over extending
When to go pressure a lane
You're against a 4 protect one line-up and you have dual cores or a core that comes online quicker
You want to force out rotations so other lanes can split-push
You want to force out rotations so others can go to roshan.
You want to force fights because you have the stronger 5 man line-up. Often in deathball line-ups with an early Mekansm on your offlaner or #4.
When to go defend
Opponents push a lane to siege your tower. Be careful not to take the bait with your entire team immediately.
Opponents push out lanes to split push
Opponents push out lanes to make a roshan attempt
Enemy supports are rotating to a lane to gank
Enemy heroes rotate to pressure one of your cores
Don't go defend if you can't hold the tower. Also don't go defend if you have no mana to do anything after your TP.
When to trade
You can't defend the tower/hero in the lane they are pushing or ganking
The enemy is going roshan and you can't contest it/you're going to be too late