Be careful not to use markers that smear, or you may end up getting marker ink all over yourself and the merchandise. If you're conducting the sale with two or more friends, make sure each person adds their initials to their tags, and then collect the tags from each sale you make. That way, you know how much to divvy up to each participant at the end of the sale. Large items can be merchandised below tables, but make sure you don't tuck them so far back that they aren't visible. Consider adding signs that point out under-table items.
People love looking through bins for a bargain.
Boxes, laundry baskets, and plastic storage boxes make good bins for small stuff. Grab some paper or poster board and make signs for your tables or to hang overhead in the garage to help direct customers to kids' stuff, women's clothing, household items, and so on. You can also easily fold stiff paper to make stand-up signs to place around the sale tables. Use good lighting if you're conducting your sale in the garage. Have an extension cord plugged in and ready to go for any electrical items you're selling.