One thing I try to have with me when I head out grocery shopping is a simple little calculator.
You can use it for one of two things, 1. to calculate the cost of each item to determine the best buy or 2. to keep a running total of what you are spending.
When I say calculate the cost of each item, I'm talking about what each serving or item costs per package. Why is this important? Sometimes two boxes will look the same, same price, same shape. But one box holds more (you can check out how many servings and the serving sizes on the side panel). You would get more for your money with the box that holds more.
You can also use the calculator to have a running total of what you were spending. I don't factor in tax, but I don't include my coupons either. That way, when I hit my budget limit, I know I come in under when I hand over those money saving pieces of paper!
I don't always do the running total though. Most of the time, I will only use this if I have a very specific amount I can spend.
Though if you plan your list before you go, you shouldn't have to worry about going over budget! The calculator helps you make those little choices that can save a few pennies here and there. But as you know, those pennies add up!