I love to yard sale, but the gas has gotten too high. So I wait for town wide yard sale days. An entire village will have one massive yard sale, or at least there will be many individual sales. Better for gas. Please take lots of small bills and change, but scatter it about your body. Some in this pocket, some in that one. That way, if you talk someone down from $5 to $3 you then don't hand him a $10 and expect change. Please don't insult the item to get a better price. When you park your car, don't block a driveway. If you are looking for children's toys, bring some batteries and small screwdrivers to test them out.
I'm sure I'll think of more...
ETA: write out the sizes of everyone in your family that you would be buying for and carry that with you. Bring a tape measure to check clothing. If you have a smart phone, set up the Consumer Protection Safety Committee's webpage on it. Check items for recalls, if you can. No sense buying a recalled item.
I find that yard sales run in two different mindsets; either they are trying to clear stuff out or they are trying to make money. If they are trying to clear stuff out the prices will be reasonable. If they are trying to make money, then the prices will be high. Watch your local newspapers yard sale listing, Craigslist page, etc. The places that have perma-sales, every weekend they have a sale, are trying to make money. They probably clear out other people's clear it out sales (or Freecycle) for their stock.