Our closet's a little jam packed but we hate the idea of devoting a whole day, even a few hours, to going through everything. Here's an easy tip we've started to put into practice to make it easier to figure out what you actually wear vs. what you think you need to keep...
As you wear your clothes, rehang them so that the hangers face backwards. (You can also spend the time to rehang all your clothes with the hangers facing backwards and then put back clothes with the hangers facing forward. Either way works.). After few months you'll have a quick visual that will let you know what you're actually wearing and you can discard the clothes that haven't made the cut.
Other closet posts
[Image: cathyse97's Flickr, with a Creative Commons License]