Don't do the boot sale on your own, take a partner. This is handy when setting up, as you will be bombarded with other sellers taking a nosy in the back of your car and stuff tends to go missing. Also when you want to go for a wee, they can look after your stall.
Also, when you park your car and set your table up on one side, make sure you lock the car doors on the other side. Keep your float of money either on you or somewhere safe.
Have fun and make loads of cash. (:o)