When shopping for home decor at garage sales you will have to keep your eyes open for items that will enhance the look you are going for. There's no point in picking up a wall sconce, even if it's only a buck, if it doesn't match what you are going for or looks like it's from the wrong era. The stuff is out there, but you'll have to be patient in finding it.
It helps to scan the ads before heading out so you don't waste time looking at sales full of baby stuff or antiques (unless that's what you're going for). Sites like GarageSaleFinder.com and craigslist are completely searchable, so you can plan your route to hit the sales that have accessories or furniture that you're interested in.
Particularly with furniture, go early. Furniture sells FAST.
Angela Sullivan gives some great advice to readers of her blog:
Look for quality. Many homeowners are just looking to clear out their basement or garage, and are selling items that are otherwise valuable. Others just have a bunch of miscellaneous items that could easily be thrown in the trash. Try to find that diamond in the rough.
She's really nailed it. You have to really look at the items you're buying, there's no returning an item if it doesn't work or doesn't fit the space. Fortunately, at pennies on the dollar compared to retail, your risk is minimal and chances are you'll be able to find something that will add that extra bit of flair to your rooms.