How do I display stuff?
There are lots of ideas for this - start by looking at the premade jewelry display from sources like NileCorp, but also hunt around for more unconventional displays, like old screens, frames, plates, etc, to add some whimsy to your set up. I'd advise staying away from anything that is unsteady or breakable or that can't handle a little water as you often have weather to contend with.
How do I prevent theft?
Stand, greet everyone, be present and attentive without hovering. (Tricky, sometimes.) And smile!
What about getting people to check out Etsy?
Hand out lots of business card with your etsy shop name printed on them.
How do I take breaks?
If you don't have someone who can go with you to help out, you may be able to booth-sit for the booth next door to you for each other to take breaks. Some larger show also provide booth sitters for this purpose.
A few other tips: bring different price points, esp items under $20, they are easiest to sell. Bring change, take credit cards. Have really good lighting...and smile, smile, smile :) Good luck!