An article by Lisa Anderson, Give Yourself a Break, describes some interesting psychological research about what causes people to experience regret. It's a good read with a couple of surprising moments. For example, it turns out that regret over things you didn't do lasts longer than than regret over things you actually did.
Another surprising revelation is this offhand comment about iPod:
Kivetz's research indicated that even if certain people can afford an iPod, they feel guilty about buying one outright."
Hmmm, well, applying your newfound knowledge about regret, you should certainly buy one anyway. If you don't, you'll regret it for years. You can thank me later.