I've only had limited success with yard sales, but here's my suggestions:
1. Try to get other people in your neighborhood or street to also hold sales. You'll get more shoppers if you have a larger sale.
2. Put up a lot of signs to direct people to your sale. If people get lost in your neighborhood, they'll just leave.
3. Be set up and ready to go at least a half hour earlier than advertised. There are always early birds who show up.
4. Make sure that everything is marked clearly with a price. Be reasonable in your prices, but be prepared to bargain. Offer deals if they buy more than one item. Everybody loves a great deal :)
5. In my experience, the only thing you can get good prices for at yard sales is children's items (toys, strollers, car seats) and tools.
6. I've read that you should have coffee available to make your shoppers stay longer and look around (but then they may just hang around longer than you want).
7. Lower your prices as the day goes on.
8. Smile and be friendly. Make people feel welcome at your sale. Encourage them to ask questions and make offers on your items.