You really want to evolve his claws first because that sends your early game damage very high when making use of your passive and singling out opponents. Next evolve you spikes to allow for greater farming and if caught in a cluster you can gain a lot of health back because of the newly shaped cone. Finally evolving you wings will allow for either a surprise jump from a distance on an opponent who is alone (as signaled by you passive) which give bonus damage on the kill or to make a quick escape over walls or just jumping a distance away.

In most game you will probably not make it to the point where you need to trade out the wriggles lantern because you should be dominating by then but if you do trade it for a bloodthirster, you will quickly be able to fill up the stacks with you evolved spike which can wipe out large minion grouping in one blow.

If you find a time where you are just being killed to quickly i suggest trading the trinity force for whatever items suites best for the situation that will help you survive longer.

Your passive works best when the enemy cant see you (say waiting in the bush before minions spawn) this gives your next attack bonus damage and slows the enemy. After you have slowed them using your Q will deal massive damage if they are isolated and will either allow you to obtain a kill of send them back to spawn to fall behind.

-Isolated Enemy's
The Best way to go when you find a person isolated from thier team is to jumo to them (with your E) and get your passive slow off giving you time to deal massive damage with your Q and follow up with you spikes (your W) if you need another slow to finish them off pop your ultimate which refreshes your passive allowing for another slow and another Q because it has a very short cool down. Your Ultimate give you increased movement speed which allows you to catch up to fleeing enemy's and you can cast it twice if you are every lead into a sticky situation.
-Team Fights
The most effective way I've found to go into a team fight is to start with your ultimate to ensure you are not focused down right away, then jump to either the carry or support because they are most likely the weakest of the fight then get a Q and pop your ultimate again to refresh you passive. If you are really losing health use your W which will restore health if you are near the explosion created from it, then get another Q off and once they are dead take a look at what is going on in the fight and choose to either stay and fight the next person in line or jump away to fight another battle.