I too suffer from this, and I'm a librarian. We're supposed to be organised. Hah. From personal experience I offer this advice: once a week, fortnight or month, whatever your personal comfort threshold is, pick a box, go through it, and get rid of anything you really don't need. Find a place immediately for this things you keep, and put the rest in the car/bin/etc immediately. Don't let the rejected things sit around and plead with you. This method is working for me, and I might have the house clear by the turn of the century.