Take a friend / partner even if only while you set up. Helps to have one guard while one sets up.
Take change and always keep money on you not in a marg tub on the table.
Don't trust pasteing tables!!!!
Load up the night before, 6am in the dark is not the best time to fall out because half your boxes won't fit in and DH has just dropped the one with the china in.
Take chairs unless you have a estate, perching on the back of a ford escort for 5hrs is not fun.
Even if you don't price things do think about what sort of prices you are aiming for and discuss with whoever is helping you, means they don't sell your rare vase for 50p when you nip to the loo.
Haggle! Start with a high price you might just get it, if they look ready to walk away you can quickly go in with a lower price. To often I hav said how much and they have quickly agreed leaving me thinking they were ready to pay more.
Be prepared to cut prices as the day goes on.
Take some polythene sheeting or at least a few bin bags to cover tables during showers and to put on the ground under cardboard boxes.
I have used a parasol to keep stock and me dry, works quite well but you might need an extended pole so you don't poke people in the eyes. Fill base before you go as you won't get a chance to go off and fill it. Sit in directly behind the table. Keeps you and the stuff in the middle of the table dry.
Take Food and drinks as coffees from the catering vans soon add up and you can get quite chilly even at this time of year.