Are you buying at a dealer, or private party?
If you buy from a dealer (and are financing), get pre approved at your bank. At least this way, you already know what interest rate you can get from them, and you can put it in your back pocket so to speak. When you go to the dealer, only negotiate in terms of vehicle price. Tell them you have been pre approved at your bank, so you aren't interested in negotiating monthly payments or that sort of thing, only out the door price. Once you have agreed upon a price, you can then tell the dealer that you have been pre approved at your financial institution, but they may try to beat the rate if they would like. Give them a chance to see what they offer you. If they can beat your rate, it saves you money, if not, stick with your bank.
One last thing I do is to take a financial calculator with me. I typically keep it in my pocket until the end where they offer me financing. If we have agreed upon a price of $20,000 for example, and they tell me they can offer me X.9% interest for 48 months, and the payment is $XXX, I will then pull out the calculator and check for myself just to make sure the numbers they are telling me are correct. If their monthly payment is higher, then either more money has been added to the purchase price, or you aren't getting the interest rate they told you.