Put your best items on most prominent display this will make people stay a little longer.
Do what the mall stores do, and dont have a clock around (unless youve got one to sell!) again, this will encourage people to linger longer.
Offer assistance, certainly if people look like they need it. But dont be overbearing. A good rule of thumb is to give your visitors a few minutes to settle in before you approach them. Then wander over to inform them youre available if they need any assistance, and fade into the background until needed.
Definitely give your best items a polish or clean, to show them off to best advantage.
Make sure your garage or yard is well lit, so your customers can easily see items of interest.
Keep the bargaining process fun. Have a smile on your face when haggling, and remember the stakes arent that high. Youre not bargaining for cattle properties here!
Give your garage a carnival, or olde world, feel, even if the garage is normally drab or brand new. Hang some dried flowers, or coloured streamers. Make up a rustic looking garage sale here signs. Play some old tin-pan alley music. Anything to make passers by want to come in to see whats going on!
To spice things up, have a red light sale every 20 minutes. Have a flashing red light handy, and every 20 minutes turn it on, and announce that for the next 5 minutes everything will be half the marked price! This may well encourage people who were wavering on an item to buy. Remember, 20 minutes is only a guide try to make sure the people who were in your garage for the last red light sale have left before running the next one otherwise this technique will wear thin.
Be flexible. Listen to buyers concerns about Price Your Items or the item theyre thinking about purchasing, and see if you cant meet them half way or even through in another smaller item as an incentive to buy.
Be yourself! (and get to know your customers): Buyers can sense when others are being genuine, and are genuinely interested in them, and respond accordingly.