Well, I've started garage saling since DD was born, so I can offer what appeals to me as a buyer, would that help?
Lay stuff out on tables. I don't bother digging through boxes. OK, a box of books or records, but that's it.
A small box of really cheap toys (10 cent or free) left on the ground will keep my daughter entertained, which means I'll shop longer. If she falls in love with a toy, I don't mind paying the 10 cents. I don't like to let her browse through the regular-priced toys, because she'll invariably choose the most broken-down, over-priced toy as the one she can't live without.
Clothes must be sorted by size. Boy/girl is good, too.
Put the large pieces, especially large children's items (yard toys and such) out in front, so they're very visible. I usually drive through a neighborhood and decide whether or not to stop based on what I can see from the street.
Write a detailed ad for the paper. I don't have time to visit every single "garage sale Saturday 7-noon" ad listed. Give me details of what you have if you want to persuade me to visit. Listing the nearest major intersection helps, too.
Hours - most I see run Fri and Sat 7 - 3 or so. I wish they'd run later, because I work on Fridays and with DD's nap on Saturday I only get a few morning hours to shop. Can't figure out for the life of me why people don't open on Sunday afternoons, but they don't.
Either sticker prices on, or know what you've priced things. Don't make me tell you that this piece came from the $1 table and that piece from the $2 table. I usually don't remember where I picked it up, anyway.