Try to line up your attacks with Vorpal Spikes so that they kill minions and harass enemy champions at the same time.
If you're having difficulty feasting upon champions, try eating minions until you're larger.
Using Rupture on creeps in combination with Carnivore is a good way to gain Health and Mana.
Purchasing a few HP items decreases the chances of Cho'Gath killing you quickly.
Focus on preventing Cho'Gath from reaching his max size.
Rupture has a smoke cloud indicating the area it will strike. Try to watch out for it to prevent Cho'Gath from being able to combo his abilities.