Arrange to borrow card tables to display items, if needed. You will need a clothesline or racks for clothing and probably extra hangers. Hanging clothes keeps items from being unfolded and jumbled in a pile. It's useful for shoppers if items are grouped. Place children's clothes or toys together for instance, and kitchenware in another area.
Boxes keep small items from getting lost among other things.
Allow plenty of space for people to move about and browse. Crowding causes confusion. If you have quite a bit of clothing, you might consider partitioning off a corner of your garage for people to try on clothes. Provide a mirror. Do not allow them to enter you house.