You will need two of you! At some point you will need the loo and you'll need somebody to man the stall for you. Take loads of change. Take loads of spare carriers if you have them. Wear a bumbag and keep all your money in it. Watch people's hands. Watch out for kids nicking stuff from under the table. Price everything sensibly. Nobody is going to want to pay more than 50p for a book unless it is a hardback in good condition in which case maybe £3 at a push. Same goes for clothes etc.
When you are packing the car, put the trestle table / stand / rugs on top so you can get them out first. When you arrive, don't start to unload straight away. As a pp says, re-sellers and dealers follow the newly-parked cars around and start rummaging in your stuff before you've even unpacked properly. They are after jewellery, silver, clocks, vintage handbags, bikes etc. Get there, park and go and buy a coffee and a bacon roll. By the time you get back to your car, the melee will have moved on to the next set of cars that have arrived and you can unpack in peace.
Don't hold anything back for anyone if they say they want something and will come back later for it. They probably won't, and you will miss out on selling it to someone else.
People haggle like crazy. Don't let it faze you.
If you can, go to one the week before and look around at what people are charging, so you can price your stuff right.