I do as much as I can to stick to budget! Usually about $100/week for a family of 3 plus some pets. Three pieces of advice that have made sticking to my budget a lot easier:
It's definitely easier when I clip coupons, both out of the Sunday paper and online. Check your usual grocery store's website to see if they have electronic coupons!
I buy things when I need them (on sale or not), but if I see my favorite products on sale I try to stock up. It might mean spending more on this trip, but it means we'll spend less in the long run.
Also, plan meals. I don't have a really rigid plan, but if I know I'm shopping for 2 weeks worth of food, I'll make a list of 14 dinners we commonly make and then make sure I buy the ingredients for those meals. It saves on my picking up a bunch of random stuff we don't need, and finding other things I need to run back to the store to get.