If you're going through your old books, clothes or DVDs to sort out what you want to keep and what you want to sell, there are usually items that you're on the fence about. In those cases you should put those items in a box labelled "Sell by X", where X is a date six months or a year into the future.
Personal finance weblog The Simple Dollar advises that as you remove an item to use it, add it back to your regular collection. At the end of six months or a year, take those items that have never been removed from the box and sell them.
You'll probably find that you never use most of the items, and selling them will result in less clutter and perhaps a few extra dollars.
Put a "Sell By" Date on It [The Simple Dollar]