Or breakfast or dinner or even a snack. Just don't go shopping when you're hungry. I'm sure you've heard this in reference to the grocery store, but it's actually true for other kinds of shopping trips.

"I know it sounds ridiculous, but we do know that, when you are hungry, you end up buying 40 percent more," says Martin Lindstrom, author of Buyology: Truth and Lies About Why We Buy. "And not just food," he adds. Hungry shoppers "actually buy more flat screens, or more furniture, or more software too."

That's because we're hardwired to crave, buy and hoard when we're hungry, so items you wouldn't ordinarily consider start looking irresistible. Of course, there's also a less-scientific reason to eat before you shop: Eating out usually costs more.