Go to one or two CBS's for a recce and speak to some stall holders to find out what time you need to arrive, how profitable it is, what's the price of a pitch and how big is a pitch. Remember that some sites charge you extra for a trailer. Outdoor sales are usually a bit more flexible regards to turning up slightly later. Be aware of the total amount of space. I once drove 30miles to a sale arriving at 7am to get the last space, five minutes later and I would have been to late.
The most important rule is to pack your car with your tables on top of your goods or on a roof rack as you must assume that you will be swarmed by traders the instant you arrive, they can be very insistant about helping unpack, and without your tables already erected your goods will be stacked all over the ground.
Pricing - I tend not to bother marking prices on items in advance and try to judge levels during the day. Many people can be remarkably reluctant to barter so if someone asks a price instead of replying "Five pounds" I usually say something like "I am hoping to get five pounds".
