My rules:
#1 - Never buy from a dealer. Private listings are the way to go.
#2 - Do your homework. ID the make, model, and year you are looking for. Watch for a month or two to get a good idea about values.
#3 - Check the listings frequently...like every thirty minutes. Deals pop up, but they never last more than an hour. On my last 4runner, dealers were asking around $10k for the generation I was looking for. Most of the models on the lots were sketchy, with body issues, obvious motor problems, etc, and the dealers still wouldn't negotiate. I found a listing for $5,000 on KSL, immediately called, found the truck to be in better condition than any of the others I looked at, and I owned the thing an hour later. You have to jump fast. Sometimes people simply don't realize what their car is worth.
#4 - Always have cash on hand when you show up to look at the deal you have located. Start low if you want, but have enough cash to match the asking price. Sellers have a hard time turning down a stack of 50-100 $100 dollars bills when you are flipping it in front of them.
#5 - Carry a gun. You are carrying a stack of 50-100 $100 bills!