Be prepared for people checking out your stuff and trying to haggle you down, even as you setting up. There tends to be a few hard core car booters/dealers hanging around first thing. I found these a nuisance, but not too bad.
I did a few boot sales and found it be ok. Some people try to beat you down on price but most were fine. I found that books/dvds are 10 a penny , so set a price of about 30p each/4 for £1.00.
I dropped my prices for some stuff I wanted rid of, especially near the end of the day, or if it wasn't a ludicrous offer. I also took a wallpaper pasting table to lay out my "wares", lots of snacks and flasks of tea.
Don't take any shit with "premium" items. I had a guy keep trying to haggle me down on a new, unused electric drill I was selling, telling me he could buy one brand new for the same price in B&Q, (he couldn't), I told him to go to B&Q then. Someone bought it later, I knocked a couple of pounds off, but wasn't giving it away like the first guy wanted.
Most people were ok and just like a bargain, no one was really pushy or unpleasant, not even power tool man.