I have read this entire thread, and there are a lot of good suggestions here. Some displays are awesome, and some need a little work. I have been doing fairs and festivals for 14 years, and these are my suggestions, based on my own experiences. If they don't work for you, keep trying everything until something feels right.
1. Don't use busy fabric for table covers. Your product can get lost.
2. Make sure table covers go all the way to the floor. Looks more professional.
3. Don't use all one color table covers. Mix it up and people will come to see what the color is all about.
4. Use different levels for display. Adds interest.
5. For those of you who do clothing, elevate your tabletop from the back. Use a piece of plywood the same size as your table and add blocks in the back before you put the table cover on.
6. Hide all the things that are not part of the display. Table covers to the floor make great hiding places.
7. Have a really good sign and plenty of business cards or promotional items. If you use promos, make sure it isn't something they are going to toss on the way home.
8. Make sure your display isn't too cluttered. If people can't see what you have, they will leave in frustration. also, be sure that you have enough on your tables or in your booth to attract attention.
9. Avoid white table covers, if you can.
Also, here are some photos of displays that I have tried over the years.
use of pegboards
use of louvred doors
table displays, showing color schemes and multi-level displays
Hope this is helpful to someone.