If you feel there's something you must have right away, there's a good chance you're experiencing a momentary lapse in judgment. A good way to determine whether or not you really something is to enforce a specific waiting period.
Photo by Jacob Bøtter
Carl Richards, over at the New York Times, suggests that an easy way to keep yourself from spending money on stuff you don't actually want that much is to enforce a "mandatory holding patten". If you think you really want something, instead of buying it immediately you put it on a list. In three days, revisit that list and if you still want the item you can permit yourself to purchase it.
This is something I do, personally, and find it very effective. I racked up a $US10,000 credit card bill a few years back because I was so irresponsible with impulse purchases and tried this strategy. I cut down on my spending significantly and was able to easily pay off the bill in a couple of years. It worked for me because I knew I'd still be able to have it in a few days if I really wanted it. Generally I'd just forget about the item entirely. Even if you're not as crazy as I was, this is still a really effective way to gauge if a purchase is really worthwhile.
Four Ways to Stop Gorging on Gratification | New York Times