Collecting Items for Sale
Start in the family storage areas: the garage, closets, shed - anything that's still in good shape but just not being used should be considered for sale. You may want to take some time to have a cleanup day, dusting off and testing anything you're considering getting rid of. Electronics and other mechanical equipment should be in working condition before being sold, and if it's broken but still useful for parts make sure you label it as such.
Give everyone in the family a box for their own rooms. Let the kids know that any of their old toys or clothes they're getting rid of will turn into money they can keep. A yard sale is a great incentive for helping your kids get the latest toy they've been asking for.
If you know you'll need help from neighbors with running your yard sale, offer them the opportunity to sell some of their own items. You'll get more merchandise to browse and give incentive for them to lend a hand.
Make sure everything is clearly marked with a price and if necessary a description. If something is missing parts or damaged, note that on the item and be prepared to discount it appropriately. Colored stickers with prices or easy-peel labels are good ideas, as no one likes sticker residue on their newly acquired item. If you have a lot of little odds and ends, try organizing them in an "everything's $1" flat-price box for easy selling.