I'm a very uh...indecisive buyer. I'll only buy something if I know for sure i'm going to use it and that I won't regret purchasing it. For instance, i'll only buy a DVD if i'm going to watch it a few times at a few occasions. If i'm only going to watch it once, I may as well have went to see it at the movies.
For clothes, I follow the same guideline. I'll only buy them if i'm going to wear them, so you'll see me standing there holding a shirt for 5 minutes straight thinking about when i'm going to wear it :O
It's a bit of a curse at times, but at least i'm not wasting any money which could be better spent. When I save for games, i'll start saving one or two months beforehand, and i'll put buying other things on hold until i'm sure i'll have enough money for both. (and that i'm sure I want to play this game)
The only sort of expensive thing i've never worn after use would be a pair of shoes. I wore them once, and then not after that. I'm not sure why. I can't wear them now because they're too small, but oh well.