If you're on the fence about getting rid of some of your clutter, put it in a box and write down an "expiration date" for the box. If the box remains unopened by that date, donate its contents to charity or throw it away.
Author Elaine St. James explains how she deals with clutter in Simplify Your Life:
Put them in a box with a label indicating a date two or three years from now -- but don't list the contents on the label. Store the box in the attic or the basement, or wherever is convenient. Once a year, examine the labels. When you come across a box whose date has passed, throw it out without opening it. Since you don't know what's inside, you'll never miss it.
St. James's suggestion to throw the box out seems a bit wasteful -- if you think some of your items can be used by someone else you should give the box to charity. You can adjust the expiration date to what you think is appropriate, or how important you think the items in the box are. The idea is once you forget what's in there, you can safely give it away with little thought.