[Halcyon]

As others have said pack a big table, a wallpaper pasting table is normally good as it folds up and is quite long.
A big ground sheet is also worth taking for items that don't fit on the table.

Get there early so you can set up. Arriving late means you don't get the best spot and you also have to try and setup in the middle of stalls and people. A nightmare. So arrive early.

As Marge mentioned make sure no one gets in your boot before you are ready to sell.
Whilst you are setting up it's often other dealers that will be jumping on you to get something they can resell later. They take advatage of you setting up and not focussing on prices and letting them walk away with something good for cheap.
So be strong and say "I'm not open for selling as yet".

Apart from that, it's a load of fun.