I don't have a HUGE clutter problem, but I do tend to collect some stuff. One way that I've found to make myself get out that stuff to donate is to make an appointment with one of the charities that picks up from your house. Around here it's the Salvation Army or the MS Society. Once I've made a commitment to them I feel like I have to get the items out. I don't end up rethinking and putting them back on the shelves or in the closet.

I'm a terrible bookaholic, but about once a year I go through all of my books and ask myself if I'm honestly going to read that one again. If I can't say yes, then out it goes, either to a charity or the local library.