When I was in high school, one of my favorite things to do on the weekend was to look for nearby garage and estate sales (yes, I was a geeky gal, or ahead of my time as I'd like to think). I love the whole idea of "one man's trash, another man's treasure". It's an on-again, off-again hobby I've kept up over the years, but this summer has definitely been more of the on variety.
When my sister came to visit in June, we went on a yard sale binge, and now every Saturday morning I'm free, I'm on the lookout for garage sale signs (husband, toddler and, of course, big car in tow). My pet peeve is when someone advertises their sale as an "estate sale" when there are only some meager offerings in the driveway. My designer's eye appreciates bold, informative signage, and I've found that usually when someone puts the effort into good signage, their sale tends to be better. That's tip #1.
My next tip would be to haggle with humility and don't pay until the very end. Gather up all the stuff you like and then negotiate your price down. After living in Beijing, I tend to offer half the asking price, but it's not a hard and fast rule (beware of lowballing and offending the seller). It really depends on how fair you think the person is being. If you're offered a great deal, don't get greedy!
While we're on the subject of "don't gets", DON'T get suckered in by stuff you don't really need, or you yourself will soon be hosting a garage sale to get rid of all the junk you've accumulated. Trust me, it's really, really easy to do this.
Tip #3 is that the early bird doesn't necessarily catch the worm. While "everything" is available at the beginning of a sale, by around 10-11, many sellers are getting hot, tired, bored, etc. and are willing to give steeper discounts. You can definitely find better deals than on Craigslist if you're willing to do the physical legwork of hunting down sales and sifting through junk. (**this tip doesn't really hold for professional sellers at swap meets and flea markets; they are much harder to bargain with**).
As a techy geek, however, I love how Craigslist has brought the whole barter/sell thing to the comfort of the home. In fact, my last tip is that if you're looking for something very specific, Craigslist will still be the best option.. And if you're willing to drive a bit further (1 hr vs. 30 min), you may get a better deal. Just make sure the cost of gas is worth the deal. I've been using a free app for the iPhone and iPad called Craigs'list by escargot, and I LOVE it! You can do multi-city searches, save searches and favorite specific ads, and in the iPad version you can view results on a map view. Enjoy!