Here are a few of my tips.

Research the car you want to buy, see if it has any recalls and find out if they have been performed on the vehicle you are looking to purchase.

Also research the value of both your trade in (if you have one) and the value of the used car you are looking at. they price vehicles with the idea that the person coming in is going to haggle. Used cars have more room to haggle than a new car. If it's been on their lot a long time you have an even better chance of getting the price down as cars on their lot aren't making them money.

When it comes to finance and they start with their third party extended warranty sale ( they make money off that) make them lower the interest rate so that it wont affect your payments. When they refer people with good credit the lenders give them a sort of kick back in the form of coupons that they can use to get interest rates lower when trying to make a sale.

Don't be afraid to walk away. Don't do the "what do you want your payments to be" because they can just play with the numbers like extending out the loan agreement to get you in but paying a higher interest or with money down.

Never accept their first offer, they will present you with a paper asking you to initial or sign that you agree. It's okay to say no and make them go back to the finance guy and try to get a better deal.

You have all the power so don't give it to them by signing the first thing they bring you.