Imagine your self making a sale, how are you going to record it, charge tax, accept credit cards, make change and package the purchases. Figure all this out in advance so you have time to get a Square or PayPal swipe for credit cards and do a trial run with the software. You don't want to make your customers wait while you try and figure it all out! Make sure your devices are charged and you have an extra back up battery and you have no problem with internet connectivity. Plan an area in the booth where you are going to conduct the sales out of the way of others who may be browsing.
If you aren't a seasoned art fair exhibitor, mock up your set up before the show. Make sure you have everything you need tent, walls, signage, and weights to weigh down the tent in case of wind. I'm not kidding about the weights don't take any chances. I heard a recent story from a friend who attended and art fair recently that a strong wind carried an artists entire tent and work into a nearby stream and they watched it float away! Plan for in-climate weather if necessary and your art is protected from wind and rain.
Make sure all your gear and your art fit in your vehicle if you have a loving spouse, (luckily I have one of those!) or a friend take a second vehicle if it doesn't fit in one. Survey the area if you can ahead of time and get all the information from the fair organizers on set up and tear down. It can be very chaotic at set up and tear down, the more prepared you are the less stressed you will be and the smoother it all will go.
Don't forget promotional materials, business cards, brochures, information on workshops if you teach them, a guest book to collect email addressed for your newsletter if you do one and last but not least, if you have the space some fresh flowers are really nice! Thanks to my husband for suggestion that one! Organize all your packing materials during set up and stow them away so that everything is ready and it's where you need it for tear down. Also bring a tool box and duct tape, you would be surprised how that will come in handy!
Here is a video of my booth set up: