This is a pretty common piece of advice, but it holds true. It's usually a bad idea to go shopping while you're hungry. You'll likely find yourself buying much more than you would if you had just eaten. Even if you're someone who is pretty good at sticking to their shopping list, there's no point in adding more temptation to the shopping experience. Instead, just grab a snack or cook a quick meal before you head to the grocery store if you can.
Of course, there will sometimes be times when you're not able to do either one of those and have to go shopping hungry. But, if you keep this little tip in your mind, you might be able to avoid overbuying.