Check Out The Clothing
One of the most overlooked items at yard and garage sales is clothing. When buying clothes at yard sales, check each garment carefully. Obviously they'll more than likely be somewhat worn in quality, but make sure there aren't any rips or stains. Remember too that clothing will be a little shrunk from being washed repeatedly, so that "XL" t-shirt may actually fit as a "L". If a lot of clothes are for sale, ask if you can fill bags with clothes and pay a set price per bag.
Ask For Other Items
Ask if there are items for sale not on the lawn/in the garageSometimes people running yard sales will have furniture or other items in their house for sale that they haven't brought out. Often they'll post a sign saying what's for sale in the house, but not always, so it can be rewarding to ask them if there's any items for sale not out on display. I once got an incredible price on a great desk by asking the owner this very question.
Always Double Check
Check and double check items you're buying. Obviously you're buying used goods at yard sales, but it doesn't hurt to be careful about what you're buying. If an item is in a box, check the contents. If you're buying CDs or DVDs, make sure the right disk is in the right case. If you're buying glassware, run your finger along the edges carefully to see if there is any broken jagged chipping. If you're buying children's toys, be particularly careful: also make sure there aren't any broken pieces, or any small pieces that could fall off. It certainly doesn't hurt to find out if a particular toy you've bought at a yard sale has been recalled by the manufacturer for safety reasons.
Go At The Last Minute
The last hour or two of a yard sale is a good time for even lower prices. People hold yard sales to get rid of stuff they don't want to haul back into their house. Usually near the end of the day they're in a much more receptive state of mind to haggle down to lower prices. Many yard sale operators automatically lower their prices during the last hour of their sale, so always ask before buying.
I hope some of the tips will be helpful to you in the weekends to come during yard sale season. May you find some great deals!