Check if table provided,
If taking own then make sure it will support the goods as basic pasting tables are famous for collapsing 1hr into sale.
Take float relevent to the prices you want to charge. You won't need coppers if you aren't going to charge 99p. Similarly a pile 1p and 2ps are no use if everything is 50p and £1, which is more usual for car booters.
Take folding seats as perching on the back of the car is not comfy.
Remember bottles of water if there is the sligtest hint of sun or warmth.
Some clear polythene or at least bin bags are handy to cover stock if there is rain.
I used a parasol on a tall pole, position the base behind the table in the centre and it keeps yopu and any stock in the middle dry(ish). I put stuff like books in the middle. Makes your stall stand out a bit too.
Take a friend, I had DH but they are useless and get fed up and want to go to Mcdonalds for a bacon McMuffin, better with a freind that you can actually talk to for a few hours, handy to let you go to the loo or for more food.
If you are on your own get chatting with neighbouring stall holders, once the vulture rush slows down they will be happy to cover you for 5m.
Find out the best time to arrive, often booters are in hours before the official time. Even if you have booked you want a good spot.
Take an box with sellotape, paper, pens, stickers, polybags, packed lunch, flask, list of suggested prices for more expensive items (for yours and friends reference), headache tablets, water, basic first aid kit (ie plasters), loo roll/ hankies, hand wash, tampax (groan nothing worse)