My rule (and I'm disciplined enough that I've never broken it) is always that I'll allow myself exactly one beer while I'm out, but only if I'm eating and intend to stay there at least an hour after before leaving. Of course, I'm like 6'2'' and 220 lbs-- One beer, even without the other precautions, is not generally going to do much to me. Anyway, this rule allows me to have a beer with my wings or whatever, but not enough to even get anywhere near a buzz.