How many times have you been shopping and bought something you didn't need just because it was such a good deal? You were saving money! Even though you spent money to save money, you just had to do it... Joe Consumer has a great post on the common ways retailers get you to buy stuff you don't need. And he's right on target with these:
Big basket = you buy more. Take the small basket at Target, and you're likely to buy less.
"Bulk" pricing. Have you seen the 3 for $12 type of deal? You could get just the one you need for $4, but you're more likely to buy three if that's what the sign is advertising.
Hiding the stuff you really need. Grocery stores do this all the time. You need the basics: milk, eggs, bread. They put it in the back of the store so you have to walk past all sorts of stuff first... and you're more likely to buy some of those things you walked by.