My sister Kim is the complete Guru of garage sales. She has one very important motto with regards to finding great pieces. It is "Old neighborhood, old junk...new neighborhood, new junk." So depending on what you are looking for...a great old piece of furniture or newer items like artwork or accessories...choose wisely what neighborhoods you plan to shop.
The main rule Kim has as far as what to pay for any given item is that you should never pay more than 10% of it's original cost. Case in point...I was out with Kim one Saturday and we came across a rope hammock. It looked like it was in fairly good shape, although you could tell it was used, and the owner said she wanted $30 for it. I looked at Kim and she shook her head...no I did not buy it, instead I went on to the next house and I ended up purchasing a beautiful wood dresser with mirror for $60. My sister actually negotiated it down from $75.
Don't be afraid to barder with the owners...especially when it is getting into the noon hours. Their stuff will just have to be hauled back into the house or to a dumpster, this will provide you with more room to haggle on the prices.
Also...when looking at items, keep in mind the possibility of refurbishing them or repurposing them. You can really make a killing and have a lot of fun with only $20 in your pocket when heading out on a Saturday morning to garage sales!