To set my budget, I used the average of several different categories from the first 6 months of this year and all of last year. For an example, this year I average $113.40 on food/groceries and $169.33 in 2008 on food/groceries. I set my budget at $120 and will get an email alert when I go over that budget for that category.
I only set budgets for categories that I can control. My utilities, rent, and car expenses stay the same for the most part and I cannot keep track of how much hot water I need to be using.