Saplings can be tossed into brush or over terrain to gain line of sight on your enemies.
Save Vengeful Maelstrom for after your opponents have fully committed to a fight.
Ranking up Arcane Smash and Twisted Advance gives Maokai powerful crowd control moves, while ranking up Sapling Toss will improve your damage output and ability to farm.
Saplings can be avoided by placing friendly minions between yourself and them. You can also just outrun them if you're fast.
Vengeful Maelstrom does more damage if you attack Maokai or his allies within the area of effect. Stay out of the area to avoid the damage.
Maokai will avoid taking damage while he's using Twisted Advance, so don't waste your spells.