my best tip is to try to make your junk look as attractive as you can, especially if you are selling stuff that has an aesthetic component. if you're selling clothes, jewelry, shoes, purses, etc., have a mirror so ladies can see how they look in 'em. hang up clothing, pair shoes up neatly, arrange like items together on trays or plates. make your set-up inviting.
this isn't important to everyone, but it is to someone like me -- if your stuff is piled up in big rubbermaid containers that i have to dig through, or if you have a bunch of stuff piled up on a tarp, i am not going to be shopping at your yard sale. take the time to lay things out neatly and make the experience as much like shopping in a store as you can. this will automatically encourage people to pay higher prices, and to act more respectfully.
speaking of acting respectfully ... if someone is polite and friendly, i will give them a better deal than if they are a douchebag. in fact i have given stuff away for free to nice people, and have refused to negotiate at all with people who get aggressive with me. i see this as one small way to discourage the douchebaggery that is so rampant in the yard sale community. :)