I second the part about the signs--very important!! I use neon tagboard with clear arrows.
In addition, price everything clearly. I use masking tape, and on children's outfits, I include price, size and if it's a two piece. All the children's clothes are arranged by size, just like a regular store.
Have a lot of change! A fanny pack is a good idea, you've got it on your person at all times. Some people have had success selling pop and treats especially if it's hot.
Good luck! It's a lot of work, but every $ earned is one more toward Disney!