I don't entirely disallow myself anything, but I find that greasy food is becoming quite disgusting - I can feel the grease on my teeth, my stomach complains.
The closest thing to a complete no-no is sugar, but I have a bit every now and then (and really enjoy it). I find that when I see sugary treats, I think about how much other food I could have for the same calorie hit, and also how much walking it would take to get rid of it.
"Some biscuits? Mmm, looks good... but I'd have to walk for an hour to balance out the calories". That tends to put me off the idea pretty quickly.