Are you going to put an ad in the paper? I remember when Alex was little, my friend told me about a garage sale in her neighborhood I checked the ad they'd put in the newspaper, and one of the items listed was Brio. I called the people up and bought every last bit of it before the garage sale even happened! If you have any big ticket things like that, you might want to think about an ad. It will draw people in.
Put a price on everything. Yes, it's a pain in the neck but some people are really shy about asking the price of anything and if they don't see a price marked, they'll pass it by. Your goal is to get rid of your extra stuff, so make it easy for everyone.
Do you have any sawhorses and old doors or big sheets of plywood to make makeshift tables? Better than laying things on the ground.
I can't stress enough how important it is to have twice as much loose change and small bills as you think you will need. Twice as much. At least.
I sold all the make-up bags that come with the cosmetics like Clinique, like Tamara. People scarfed those up. You know what else people bought? Cards. We had a ton of cards from all the times we've donated to different charities, and I marked them all 10 cents a piece and they disappeared in no time flat. Go figure that one out!
Face the fact that this garage sale is going to wipe you out, first in the preparation for it, then in the aftermath of dealing with all the bargain hunters out there. Have your kitchen well-stocked with wine and goodies because you will need it to recover. And have the pizza delivered that night.