Usage Earth Spike - When there are several enemies coming, you may want to turn around and throw an impale. Done also for initiating minor battles. Try to align yourself with the foes and click the farther target. Chances are that you'll catch him and one or two other enemies in the Impale.
DON'T CANCEL ANIMATION ON IMPALE. It may make it run shorter than it should, thereby making you miss it. Be patient, it only takes a split second to cast it fully. Can be cast up/down cliffs and for that you must click ground in the general direction of the target.
Hex - Be reasonable. Try to cancel any chanelling skills, to disable starters/blinkers/disablers and key heroes. Think outside the box and decide who and when you must Hex. Also, Hex has no casting delay, abuse it.
Mana Drain - Do it whenever you're safe to do it and still capable of catching up with foes (unless you're laning, in this case you can just MD to harass) or escaping by yourself. E.g. Enemy Sven is chasing an ally with 350 mana and casted Storm Bolt like 10 seconds ago. As the cooldown is 14 seconds, he may stun your ally and kill him. But, if you Mana Drain lvl 4 Sven for 3 seconds, he'll be left with 50 mana, what's not enough to cast Storm Bolt, leaving your ally safe. At this moment, you may get away too, help your ally further more for him to get away or still turn around and kill Sven as he's out of mana, you can easily gank him.
Mana Drain can be casted right after Impale, also can be shift-issued, but not necessairly. Abuse it inlane because it takes a lot of mana out of your enemies and replenishes your own mana. Just remember to play it safe.
Also useful at teambattles for denying low mana pool heroes to cast key spells, like Treant's Entangling Roots and Leviathan's Ravage. It does destroy illusions you Mana Drain, it takes less than a second and takes only 10 mana, so it's awesome for this.
Finger of Death - You may use it on every situation that you are not sure if you can get the kill without it, but still totally sure that you can get the kill with it.
Can be rapidly cast after Impale, just like MD, has large range and can be used up/down cliffs. Try not to overkill with it as it's your precious weapon. Also, try to leave the frag to the carries.
When to upgrade what
MD at levels 2 and 4, Voodoo at levels 8 and 9 when:
You're facing a highly mana dependant hero inlane
You're ganking a lot and need to disable dangerous heroes
Max Voodoo ASAP when:
You're facing a lot of dangerous heroes that can do well without much mana
Heroes that have magic stick or bottle / chanelling spells / you're ganking a lot
Note that you may change the skill build accordingly to the game. Just be reasonable. If you don't feel like getting MD because you barely have the chance to use it because the ganking action is really intense, so go for stats at levels 10/12 and so on. But I should advise that if you have 2 levels of MD skilled, you may stop by a lane to regen mana and later proceed with the ganking. Also, as stated earlier, mana dependant heroes really suffer if MD'ed.