Car salesman can't cheat you if you know what's up. The things you gotta know are:
1) EVERYTHING is negotiable. If he claims it's not, go to another dealership.
2) Don't get ANY "extras" like rust protection, wax, etc. You can get these far cheaper at third party places, and most of them you don't even need anyway.
3) The game is, he's trying to get you emotionally into the car (by driving it, sitting in it, making you think you have to buy today) and he wants to sell it to you TODAY more than anything.
4) Never bid against yourself. He'll say something like, what kind of monthly payments are you looking for? And you'll go, say, "$300". And he'll say "is that all?" And maybe you'll say "well, I could go up to $350". Congratulations, you just bid yourself up 50$ a months.