Demand a safety report and vehicle history report from the dealer. Any good car dealer will have these items available for any vehicle on the lot. This is information that will tell you how safe the vehicle is as well as any major accidents or work that has been needed on the car.
Don't give your SSN out too early. Many dealers will ask for your social security number before negotiations even start. They will use this information to pull a credit report. Remember that each time you have your credit run hurts your score. First have an ironclad deal before giving your SS number.
Many people who have bad credit end up at a buy-here pay-here dealership. Be very careful of these types of dealers. Usually they buy cars at auction for little money and the majority of the money they make is off of the high interest rates they charge. If you have any choice, always get a loan from a bank, not the dealership.
Don't hand over the keys to your trade in until the deal is signed. If they have your car "hostage," they can use it to pressure you to buy the new car. Wait until the deal is done to ensure you keep control of the situation at all times during the transaction.
Watch out for dealership advertising fees. These are often tacked on to new car purchases, and you should not have to pay them. Car dealerships have to advertise as part of doing business, and many shoppers who challenge this fee will win. If your salesman will not remove this fee, ask to speak with a manager.
A dealer with a great reputation may offer you a better deal than one which advertises great prices. You may find that a dealer who people like to buy from offers perks which aren't available elsewhere, including reduced pressure sales tactics and lower overall price due to freebies thrown in to the sale.
How the staff at a dealership treats you when you arrive and thereafter show a lot about how they work. If the same salesperson takes you from start to finish, you have a good lot. If they pass you off to a high pressure "closer," you might want to head to another location.