Be knowledgeable of your vehicle. Study the terms of each part. Know that particular car inside and out. Stop at the car dealership to do a recon of the particular car you would like to buy. Then study up on that make/model/year. Know this vehicle.
Don't sign anything the dealer gives to you unless you've read the paper and really understand the terms and conditions listed. Sometimes you sign away legal rights. This is very important to read and re read until you understand what it says. Don't feel pressured by the dealer. The will wait if you need more time. The sign of a scam artist, he pressures you to sign it fast. Always make copies of your paperwork as well.
Double check your paperwork. If you have a warranty, make sure the box is checked. I found this out the hard way. Express warranty is no good in the court room. They will want to see the checked box. Sometimes the dealer maybe anxiously fumbling the paperwork around and might have not noticed. Others do it in hopes you won't catch them. If you notice this, point it out to them and make it right.
Get a mechanic to come look at the car prior to the sale. Make sure you get a complete inspection especially with used dealer ships. Some cars are only worth as much as the down deposit because of the many problems that go unnoticed. An inspection will save you money, frustration and time later on.
Keep composure. Give yourself a few days to a week to buy the car. Don't be in a hurry or you won't get the car you want. Some dealers will try to convince you of a certain car because it may cost more and they get a commission on what you pay. Don't be mislead. Stay focused and alert to everything that is going on. Make sure you come with someone who has done this before so you don't look like an easy target.
Make sure your credit rating is good or you have someone who can cosign for you with a good credit rating. This can give you a better interest rate which gives you a better deal.