How do I use Sentry Wards properly vs Invis heroes?

I usually play Support, and whenever the enemy team has Invis-heavy hero such as BH, Riki, Nyx or Mirana, I buy Sentry Wards and spam them. But I found out this is not very efficient in the long run; many times I'm forced to survive on Boots and Wand until I get my Arc Boots at around 20 minutes mark.

So my question is, how do I use Sentry Wards efficiently against Invis heroes? I read somewhere that "Sentry Ward to prevent getting initiated on or for when you will be around for a bit", "Dust for ganking / skirmishes". But in most cases, I seem to spam the Sentry Wards along the road (though maximizing their coverage) just so I don't get jumped on and cut down in seconds; especially vs Nyx.

Where your heros will be the most, for example, if I see the hero is missing, I would drop one in the lane the carry is farming a bit ahead of where the wave is(or behind if it is pushed up). Hopefully, your carry can control the wave well. Also in common rotation spots. So if its a mid nyx, I would put one with the observer ward once he gets 6. Using heros as bait is a great way to kill an invis since chances are, they will get greedy.

The trick is to place Sentry Wards at the place they are likely to Gank from. It gets easier if you know how to play these Heroes yourself, so you know which route they are most likely to take to get to you and your Carry. Take the Safe Lane for example...

Most Invis Gankers will come through the Jungle from the River, because most Supports will spam Sentry Wards along the Road. So you need to have a Sentry Ward and an Observer Ward in a place where you can spot them as soon as they enter your Jungle.

Some Invis Gankers will Town Portal Scroll to their Offlane Tower for a quick Gank. This can be spotted by having an Observer Ward at the edge of the River that gives vision of their Tower. Although more experienced Players might TP to the Tier 2 Tower, further back to avoid this.

A Bounty Hunter will try to use Shadow Walk to stay where he can gain Exp, or he might use it to body block Neutral Camps so the Creeps don't spawn there and the Lane gets Pushed, or he might go after a Jungler, etc. You have to anticipate which of these actions he's trying to do, and which one will make him the most vulnerable, then place a Sentry Ward where you can see him as he tries to run away to increase the odds of killing him.

Basically, I recommend playing some Invis Heroes to get a feel for where they need to be in order to be effective. Also, try watching replays of your games and thinking about whether there was a better place for that Sentry Ward and could you have anticipated that Hero's movements in any way? etc.

The trick is to put them at the right moment: if you're pushing a tier 2 tower and there are no enemies, the enemy Mirana has most likely used Moonlight Shadow. Other common situations in which I always plant a Sentry Ward are when defending towers, fighting Roshan or dewarding.

It really isn't that difficult, just place a Sentry under an enemy tower you're pushing but outside its range so it doesn't get spotted by its true sight; same for when you're defending. If you keep getting caught by a roaming invisible Hero, just stick to your team or farm your jungle and place a Sentry Ward there (whithout blocking camps!).

I tend to only stick a Sentry Ward down where yourself or your team are going to be staying for a while. If someone is farming the lane and not really moving from that spot, i'l stick a sentry somewhere near them. Pushing a tower, I tend to leave a ward behind the team out of tower sight, as most invis heroes will circle around behind than attack head-on.

Spamming wards all along a lane, only time I would do that is if they have mass invis ( Mirana basically) and your 5man pushing down a lane but havent encountered anyone yet. Spamming just to keep yourself safe from ganks probably isnt worth the time and gold spent.
Exception is Broodmother, who is a pain to deal with at all times.

Sentries will be scattered around your half of the map if you use the "dont get initiated/ganked" rule, you just have to be cautious when your not underneath one. Against strong invis gankers with Stuns/Silences ( Riki, Nyx Assassin, Clinkz and Broodmother will build an orchid) you will want to move between sentries with someone else if possible.

Spamming Sentry Ward in lanes is a big investment, note the players tendencies, and ward accordingly, be sure to have a decent enough spread so your vision isn't inhibited, and if you can spot them, don't jump them too close to the edge of your ward vision, you'll scare them away.
Always carry Dust of Appearance when you can, and a gem is a cure all in fights but don't stop warding or buying dust considering the gem carrier won't always be near everyone.

Also, consider the hero or heroes, is it a Riki? Vision coverage for positive accountability of him, as much as you can within reason, as stated in prior posts, in common Ganking areas (common map spots, and player tendencies). The same will apply to Nyx Assassin and Bounty Hunter.
Natures Prophet will commonly build a Shadow Blade, so that's a situation you'd benefit more from dust, to be able to teleport to him and dust him, sentries placed outside of your towers sight help significantly as well for when he goes invis to Tp out.
Treant Protector can be invis by trees, so noting when he faded and where he last was are the obvious keys to finding him, and how many trees are around him.
Invoker follow the footsteps, if he's quas-wex, he will be pretty damn fast so sentry coverage would work well to keep eyes on him, while he's much slower in Ghost Walk if his build is quas exort, in which case dust would be sufficient. Note: catching him early enough may force a tornado, or deafening blast which will stunt a chase, but break invis.
Slark bloodseeker I believe is the only counter to Slark's ultimate if slark is low enough on health, but sentries in the jungles should do the trick, since he will tend to go hunting quite often in the jungle or by ancients, to pick off farmers who will be slower to react to him while focused on farming.
Clinkz you're best counter is to slow him down, deny his farm early, since his last hit animation with flame arrows is still easy to beat with denies (especially in lower to mid ranked pub games), ward your jungle proper, as well as secret shop areas, any place he can get behind you for strafe essentially.
Phantom Lancer gem of truesight and Dust of Appearance, determine the real PL from illusions, and dust him as soon he uses dopplewalk

Hope that's not a tl;dr, and helps

Caveat: the Slark entry to my previous post is intended if he's building Shadow Blade

chronosphere reveals invis and slarks ulti