There are so many things I wished I knew about when I started! Some basics would be:
Stand, don't sit. Potential customers are more likely to ask for help and/or buy if they feel you're totally available to them.
When people enter your booth, look busy if you can. Don't make them feel like you're concentrating on them, expecting them to buy something.
(In other words, make yourself available at a distance!)
Don't ask people if you can help them. They might say "no", which is often a dealbreaker in sales. Instead, say something like, "If there's anything I can help you with just let me know."
If you can be making your jewelry while your exhibiting, that often interests buyers.
Have a small dish of candy or something to offer your customers. It's a nice goodwill gesture.
Don't be afraid to charge what your product is worth, and *never* apologize for that. Make sure prices are visible--people don't like to ask.
For display, try to have different levels-use shelves for example. Use neckbusts, or if you can't afford them at first, improvise. I once saw somebody use styrofoam blocks draped in fabric, with necklaces and bracelets draped over that.
Make sure you have lots of change.
I'm sure there's lots of things that I'm leaving out. If you get really into doing shows there's a great CD by Bruce Baker about craft show dos and don'ts. I did shows for about four years and I had lots of ups and downs. If you don't sell a lot your first time, don't be too discouraged. It takes time to get good at it, and those of us who are good at making things are not usually good at selling them. I generally want to treat my customers the way I would like to be treated, but I hate it when people try to *sell* me something. Not everybody is like me, of course!
Good luck! Let us know how it goes.