Put things up on tables, as much as possible. I don't like to bend over to pick things up off a blanket on the ground. Somehow things on the ground are perceived as "less than" things on a table.
Play some music if possible. Fun and upbeat stuff. It relaxes people. If I go to a garage sale and am the only one looking and there's total silence, I feel awkward.
Greet your customers, be friendly, be helpful, but don't hover.
As a buyer, if I really want something but don't have enough cash on hand, I will ask the seller if she/he will hold it 20 minutes while I run to the ATM. I promise them if I'm not back in 20 minutes, they can assume I'm not coming, but please give me a chance. I would honor someone's word for 20 minutes as a seller, but that's it (considering we live ten minutes from several banks).
Sellers, it's a good idea to have a cooler of drinks for sale on a hot day. Once a person opens his or her wallet to buy one thing, it puts them them in a spending mood.