I either tell the cook what I can and cannot eat ahead of time, or do the cooking myself and live vicariously by watching everyone else eat. I made creme brulee last night (thank you Nigella Lawson!), and got my jollies by just smelling the stuff.
Having several recently-discovered food intolerances also helps keep me away from not eating clean - if I know that I'm going to feel physically ill after eating gluten, dairy, or some fish or nuts, I'm less likely to. It's also a good excuse when people are less than accomdating about dietary choices.