I try to stick within a certain range, but sometimes we go over. I don't let it bother me if it's for a legitimate reason, like if we have company or if I'm too sick to cook.
I find that I don't have to jump through any hoops to stick to my budget as long as I don't buy too many things that are out of the ordinary. Every once in a while I'll try something new, but not if it's expensive, unless I have a coupon and it's on sale. I try to buy very little processed food and cook from scratch, so that helps. Fresh produce and other fresh ingredients may seem expensive at first, but they're really not when you look at all the things I don't buy. I routinely skip entire aisles at the grocery store. (That cuts down on impromptu purchases as well.)