Rewards...we have sticker chart which he likes. He always love picking out a "big car sticker". He also gets a candy (leftover from CNY....). He doesn't really care about food that much and he hates ice-cream (doesn't like cold food, nor hot food for that matter). He like cars but I figure I can't really give him a toy every time he pee or poo as there are already a lot of toy cars in the house.
btw, my boy says NO too when put on the toilet a lot of the times. We let him bring a toy to the toilet, or read a book on the toilet, which helps keep him there for a bit. I also try to count with him, sing song with him (when we count to 10, or when we turn the page, I'd ask him to push -- like birth coaching -- someone should make a career out of poo coaching).