1. Don't do it. Seriously, just give it all to charity or call one of those companies who haul things away for a small fee. It's not worth the effort.
2. See above. If your wife/husband/partner is pushing you to hold one, be firm in your conviction. DO NOT give in!
3. If you have to hold one, do it big. Put the ad in the paper, put out the signs, go the whole nine yards.
4. Start it early in the morning. No later than 8:00 AM.
5. Put "No Early Birds" in the ad, unless you want people showing up at 7:15 AM for your 8:00 AM garage sale.
6. Even if you put a "No Early Birds" warning, be ready for early birds at seven fucking fifteen in the morning.
7. Have at least three other people helping you first thing in the morning. All the action will take place that first hour and you'll need the help with all the expert hagglers attacking you from all sides.
8. By 11:00 AM you'll be done. Advertise your sale until Noon and then call it a day.
9. If you have anything you want to get more than $10 for, sell it on eBay first.
10. If you have big items like furniture, auction it on eBay with the auction ending three days before your garage sale. List the item as local pickup only, then encourage the winners to come down and pick up their items during your garage sale. You might be able to get them to buy some more crap while they are there.
11. If you are selling CDs, put them in lots of at least 20 and sell the lots for $5 each.
12. Classics of modern cinema on VHS, like The Third Man and François Truffaut's 400 Blows will not sell, but people will fight over Sid & Marty Krofft's H.R. Pufnstuf tapes.
13. Finally, just don't have a garage sale.
14. Really, I mean it. Don't fucking have a garage sale.

I hope these tips help you get the most out of your next garage/yard/stoop sale.