In my mind, there are two kinds of fashion-obsessed people: the savers (keep every item, never throw anything away) and the purgers (when a new item comes in, another one has to go).
I fall much closer to the saver end of the spectrum (as in every time I try to clean out my closet, here is the conversation I have with myself: "you don't need it" I put it in the donate pile. Stare longingly at it. "but what if I do? what if I suddenly have a formal ball or (fill in other activity I will probably never do here) to go to? you never know!" put it back in the keep pile.)
So my closet is filled to bursting (see: the scene in Confessions of a Shopholic where the spacebags explode...my closet is dangerously close to that) with clothes from high school to now. Stuff I haven't worn in years. If I'm not busy convincing myself that it may come back into style/I may need it someday, I'm too busy reliving all the sentimental memories attached to that item. "Aww, I wore this to my birthday party freshman year of college" Etc. I tend to view my clothes as the memories, instead of just the physical reminders of them.
So while I will never be one of those people that donates their clothes with abandon, hopefully I can learn to better calm the "what if?" voice in my head. Or maybe I'll just move to some place with a walk-in closet.