Lots of good reasonable advice in this thread. Couple of other things I've seen or remember from buying my jeep a couple of year ago:
1) If you are getting a loan, make sure that there is no prepayment penalty. I got a loan for a portion of mine (partially to help build my credit), but paid it off extremely agressively (I think I paid it off in 18 months). I did put about half down so that helped.
2) Take your time and read all the paperwork. Get a copy of all the paperwork. I read an article a couple of weeks ago about some shady dealerships going to electronic signing on documents and people getting absolutely hosed on stuff.
3) Like lots of people have mentioned, you have the power to walk away. If you think they are playing games with you, or if they get pushy, or just make you uncomfortable, you can leave at any time. Don't get attached to a car.
4) You may have to dig for it, but find out what the dealer invoice price is for your car. the difference between list and invoice on mine was about $5k.
5) I think the thing about selling at the end of the month is at least partly true. I bought my jeep on black friday and there wasn't much to do in terms of negotiations. I came in, offered $~1.5k less than what I expected and was prepared to pay (which was still 2k under sticker), they counteroffered $500 above my offer and I took that deal.
One thing to remember is that a lot of times people are looking at the same car at the same time. Happened with the aforementioned jeep. I looked at it on Wednesday (made my decision on Thursday) and came back Friday morning to make the deal. They actually sold it that Friday AM (they did actually sell it, I saw someone taking possession). Dealer found an identical model about 45 minutes away at a sister dealership and got that one up to me that afternoon.