Understand that you will forget stuff. This is your first time, so really it's more for your learning and experience, and if you sell a ton of stuff, great!
1) Make sure someone goes with you, or comes by at various intervals so you can use the restroom and eat. A stressed and really-has-to-pee salesman is no good!
2) Take business cards.
3) Take some paper & a pen that match your labels. I tell everyone this! You can easily purchase mini photo holders (clear acrylic stands or the kind with a wire and a clip at the top) so you can make signs on the fly, and they'll be consistent with your branding.
4) Read the excellent soap queen article about getting rude questions. Because it will happen (if not now, at some point). It's better to have an answer before you open your mouth and fire comes out!
5) Set up your booth beforehand, as much as possible, and tweak it however you want. Then, take pictures and print them out or save them to your phone so you have something to go by the day of, when you're so nervous you could puke!
6) Keep a large zip-loc of weird but sometimes necessary tools - extra packaging bits, zip-ties (for windy booths), tape, scissors, bandaids, a tylenol or aspirin single use, granola bar, tampon (if you're a girl) and a water bottle. it sucks to be outside in the heat and run out of something you REALLY NEED just then! You can keep this baggie in your vehicle which comes in handy anyway.
7) Wear an apron with pockets. Keep your large bills in there, not in a cash box. The cash box should have your coin change, and enough 1s and 5s to make change for a few customers. Use a square reader on your phone to take cards. Don't forget to collect sales tax if you need to in your state - a little receipt/invoice book can be bought @ Staples for $4 or so.
8) Lastly, if it's a FM you'll be doing weekly, consider having a running batch you bring every week, so even if you run out, you can take orders and the customers will pick them up next week. For example:
You made 400 bars of Peppermint soap. However, you have 12 scents, and 4,800 bars of soap is WAY too many for you to handle on your own. So take 100 of each. Leave the other 300 at home. Should you run out, take the customer's name and phone number on the receipt book, and bring his/her soap the following week, with a reminder call the day before. It gets you their money over a competitor, AND it ensures they come back to see you!
As you get more comfortable doing fairs, consider doing on the spot shipping. Bear with me, I know this sounds insane! Keep a few prepaid priority small boxes in your booth (as of this writing, they're around $6 and guaranteed forever.) Customers who are shopping for friends and family will appreciate not having to lug things home, and you can easily print the shipping label right there in front of them if it's slow. Slap that puppy on (because you have a small bag of paper shred or whatever handy already) and mail it as you leave on the next morning. Be sure to include business cards.
Good luck with your fair. Let us know how it goes!