I got this idea from pinterest and then forgot about it...but I don't think it's too late to try it.
On January 1st...or whenever you remember...turn all of your hangers in the closet backwards, so it hooks from behind. As you wear something, then wash it and return it to the closet, hang it the normal way. At the end of the year, anything that it still hanging backwards did not get worn for a whole year and should be donated or sold.
I know there are lots of clothes in my closet that I haven't worn but I always think I just might need it someday. If it doesn't get worn in 2013 I'm getting rid of it.