Since I have this dragonish/hamsterish tendency to accumulate things, my partner and I decide that if I buy a new thing, I'll have to give away/sell an old one. Being a sentimental creature, I always find it sooo difficult to discard my old stuff. I feel they are a part of me. They help me to remember. Ohkay, not all of the memories are nice, but still, I think you get the point.
From books concerning feng shui, we learn that it's not healthy and wise to keep all of our old possessions. If you still wear/use it, no problem. Let's say you have a dress that you've never worn for years, what's the use of keeping them? All right then, you want a memento of the good ol' days. Several of them will do.
I have bought plenty of new clothes lately, so I have (painstakingly) choose the ones that I'm willing to let go. My partner and I also plan to sell/give away our old, old furniture and electronic stuff. One of these days we'll do so, to make room for a new one.