My big rule: NOTHING UNPLANNED. I may not have the perfect plan, but I never, ever give into temptation. If I am going to work a 16 hour day, I decide before hand if I can skip exercise that day. If I am going to a party, I decide before hand what I am going to eat. If I decide I need to increase my calories by 100/day, I decide to do it and then wait 3 days to actually do it.
This has several advantages:
1) I am good at lying to myself/convincing myself of things in the moment. By waiting at least 72 hours--and more often, weeks--to change my routine, I know it's based on logic, not impulses.
2) I feel more successful. I don't have to be perfect, but I hate not living up to my own expectations. That feeling that I "gave in" really makes me sick.
3) "Giving in" reinforces itself. A dog can learn to sit up and beg if it gets a cookie each time. If you give yourself something yummy whenever you are weak, it encourages future weakness.
Anyway, this is my one unbreakable rule. It's been as good for my finances as for my waistline!