1. Do your homework. www.edmunds.com
is your friend.
2. You didn't say whether or not you will be financing this purchase. If you will, then arrange for the financing yourself - before you begin shopping. This can save you $$$ because if you buy from a dealer, they often make $$ on their financing deals.
3. No matter how hot you get for a particular vehicle, be prepared to walk away if you can't get the deal you want. There are plenty more vehicles out there.
4. Never ever ever discuss payments with the salesman before you make the deal. They are pros at hiding fees/costs and rolling them into a payment that you already said you would be comfortable with. Know what price you are willing to pay and stick to your guns. The payments will work out in your favor because you already arranged your financing...you should already know what your payments will be for a given amount of $$.
5. Don't get suckered into extended warranties, rustproofing, appearance packages etc etc etc. These are almost ALWAYS dealer profits.