"I will in a bit!"
"Sorry, but I don't like (or can't eat) [insert ingredient here]."
"I think I've overdosed on X; I just can't eat it anymore!"
"I wouldn't say I am obsessed with dieting, but I am conscious of what I eat."
"I have to drive."
“If I start eating X, I'm sure it's so delicious I won't be able to stop!"
“Thanks, I am already full! The food was delicious!”
And of course the simple and usually effective “No, thank you.”