I highly recommend David Allen's book "Getting Things Done." It's about productivity, not decluttering/organization, but I think many of the strategies he advocates are useful for reducing clutter as well. For instance, if you have a partner, make sure each of you has a work station, and mail/other items get put into the correct inbox, so you are responsible for your stuff and he is responsible for his stuff, and there's no random unassigned floating "stuff."

I also go through my closet/dresser every six months or so and come up with a bag to go to Goodwill or consignment (stuff that I don't wear but is in pretty good shape), and if applicable a bag to be trashed or made into rags.

I have sold a number of things on Ebay, but mostly it's been bike parts with very little wear. I usually set a fixed price a bit lower than I actually think the item is worth. All of the bike parts I've listed on Ebay have sold. Other items have been more hit or miss... I went through a ballet flat buying spree last summer and bought a few pairs of shoes that I realized after only 1 or 2 wearings weren't right -- I sold one on Ebay to a happy customer but the other wouldn't budge. If you're pretty sure what you have is marketable, it's worth the effort -- if you're not so sure, it can feel like a waste of time and listing fees.

When I "clutter", it's usually a sign of some emotional issue I'm avoiding. But that's just me .