I'm not sure if everyone has this mentality - probably not - but nonetheless when I shop I always keep the receipts.
This is why I buy random things I'm not entirely certain I want. It doesn't matter, I can return it. What's better - that money becomes free shopping dollars.
I know - it's not really, it's still the same money coming out of my account, only it doesn't seem like it. Now it's free money not tied to my budget. YAY, let's go buy frivolous crap.
Which brings me to impulse buying. These crafty people are very smart with the items they put in creative spots (like right by the line as you're leaving). You can't get out of line and you don't have enough time to really analyze if you NEED that item.
Though, need is a relative term... you may have heard my reasoning with this before.
Then "Tada", you come home with some new and creative tool, gadget, whatever. Sometimes, this turns out to be something really awesome. You congratulate yourself on your amazing find and brilliance. Other times, it turns into a return item.
See the vicious cycle here?