Found a great article from consumerist.com! As a person who holds on to way more stuff than needed, I'm printing this out and posting it!
1. Don't do it all at once. That's how you get overwhelmed and give up before you start. Set a timeline and divide the job up into smaller goals.
2. Don't make a "maybe" pile. Everything should be a "yes" or "no." That's how you got the junk in the first place . . . by assuming you'd need things later on.
3. Have a garage sale. Advertise well, and offer volume discounts. Also put out a "free" box with stuff you can give away. People will be more likely to shop if they think they can get something else for free.
4. Post bigger items online. For furniture and appliances, the best thing is to list them on Craigslist or eBay. You'll get more exposure and usually a better selling price.
5. Use Freecycle.org to give things away. It's a website where you post stuff you don't want. Usually there's someone out there who'll want to take it and fix it up. They probably CAN'T fix it, but that's fine because now it's THEIR junk.
6. Donate to charities and local organizations. A lot of charities will schedule a pickup with you. Or you can contact places like schools, assisted living facilities, and community centers to see what kinds of things they can use.
7. Sell at consignment stores or flea markets. Do it for things like antiques and special items that you don't want to put out at a garage sale. Or you could go to a pawn shop or antique dealer, if you have anything that's actually worth good money.
8. Put it out on the curb. If you can't sell it and you don't want it, try leaving it outside. People will take just about anything. If they see it sitting there unguarded, they'll think they saw it first, and they'll grab it because they don't want to miss out on a good deal.
9. Throw it away. After the first eight steps, whatever is left goes straight to the trash. Take it to the dump if you have to, or call a junk removal service.