Playing AP Zac build, your default rush is Hextech Rocketbelt.
Adapt to an enemy team. Are they full of tanks, and they rush tons of Magic Resist? You rush Void Staff as #2 or #3 item to counter MR stacking. Their team has champs with lots of healing? Rush Oblivion Orb as #2 item (upgrade to Morellonomicon as 4th or 5th item).
Somebody is going after you each fight? Always getting ulted by enemy assassins? Rush Zhonya's Hourglass.
Tips and Tricks
Riot
Picking up goo chunks is very important to staying alive.
When Cell Division is ready, try to die in a position that makes it difficult for the enemy team to kill your bloblets.
Charging Elastic Slingshot from the fog of war will give opponents less time to react.