Think it through. In order to have the best garage sale you can have, you and your family are going to have to let go of some stuff and come together as a team.
Establish a basic premise. Give boxes to everyone in your family a few weeks before the sale. Have each person fill a box or two with things he or she wants to sell (beyond what's already piled in the garage). Let the kids know they can keep the cash from their items.
Prepare together. Scrub, wash, polish and launder anything you plan to sell. If an item needs a simple repair to greatly increase its selling price, do it. Hang clothes on makeshift racks, sorted by size.