I LOVE buying cars. My wife hates it.
First of all, I don't buy new cars. Best deals of all are the gazillions of vehicles that are part of the manufacturer buy back programs that exist with the rental companies. I bought 2005 Chrysler T&C last year for $8,000 less than MSRP.
But, sounds like you are going to buy new since you have credit towards GM. In that case, my advice would be the following:
1) Know what your max price is and don't go above it.
2) Settle on the price of the vehicle before you talk about your trade in. And know what your minimum price is and don't go below it. Better yet, sell your own vehicle yourself.
3) Walk out if you don't get what you want (as long as your research tells you that you are being reasonable).
4) Arrange financing outside the dealership. I ended up with dealer financing on our van, but they had to match the bank rate I had received ahead of time.
5) Don't buy a long term maintenance contract. The likelihood that you will need it probably will not outweigh the costs.
6) Don't forget, they are trying to make money at every stop during the purchase. Add ons, agreements, extra floor mats (why not just go to autozone, yourself?), everything. Just keep saying, "No, thanks."
7) Do your research. It's tough not to get outfoxed at the dealership. They know what they are selling and if they are any good, they know how to sell it. Part of their advantage is that they do this everyday and you do it, what, once every five years? Get on the internet, look at Edmunds.com and all those other sites to know your product the best you can.
8) Be nice. I think a lot of people think car dealers are just slime bags. The truth is, they DO have to make money from your transaction, so there is no reason for you to expect them to give you an unreasonably good deal. And I do better playing the happy but very well informed consumer than I do the crass, hardened negotiator. In a way, I think it is best just to establish with them right up front that you are a reasonable, well informed, no nonesense consumer who will buy on your terms. No need to act like a crank about it.
9) Shop around. Use the prices you get at one dealer against another. Be honest about it, because chances are they know what's out there and what other dealers are offering. If they didn't, they would not be in business long.
10) If you are shopping with your spouse, make sure that both of you know what you are doing. My wife and I establish our highest price before we go into the dealership. That way, no matter who the dealer latches onto, we know where we are going.