The first poster is right on the money: do your homework. Whether you are lookig for a new or used car you need to know what you are getting and what it is really worth. Spend the time going online and researching the features of cars in your category and price range, as well as the car's history and condition if used. Know what the competitors are offering long before you talk to anyone about a car. Use a mechanic. Use the internet. Do your homework, don't take the word of anyone trying to sell you a car, because the sale is their only goal.
Know what you are looking for and what you can and are willing to pay. Research, research, research and then look for when and where the best deals are. There are definitely certain times of the month and year where you can get a better deal on the same car.