I would have to agree with the people who say "don't beat yourself up over it." It happens to the best, and you just have to get back up on the horse/bike/wagon-to-oregon or what have you.
Personally, I allow myself one "cheat day" a week. I maintain a very healthy diet for six days out of the week, but on that one day all rules are out the window (well, I do maintain my vegetarianism, but other than that...) Honestly, it just reinforces my healthy eating habits because I usually feel like crap after my cheat day. The last cheat day, the only REALLY bad thing I ate was a slice of sweetpotato pie with a scoop of wattleseed ice cream.