There are lots of tips in the DEALER SERVICE section on this, but here are some of my thoughts on the topic:
I think that
www.carbuyingtips.com is the most concise expose on how to buy a car economically with a minimum of hassle. You don't have to follow every single step exactly, but in practice as long as you understand the basic philosophy, you'll be okay:
1. Get accurate and up to date invoice pricing on your desired vehicle. Fightingchance.com is supposed to be worth the $, but I just used several sites such as edmunds.com, carsdirect.com, etc. as a comparison.
2. Go through the Internet/Fleet sales departments--don't walk in to the dealer and expect to get anywhere with that wolfpack unless you're very sophisticated, very crazy, or both.
3. Get your own financing first and see if they'll match or beat it.
4. Try to sell your vehicle first so that you have one less price variable to deal with during your negotiating process.
5. "Don't shoot until you see the whites of their eyes!" This site is a pretty good resource for about what you can expect to pay, so when you're ready to pull the trigger, THEN politely email as many dealers as possible telling them you're set to buy a vehicle in the extreme short term, are looking for their best Internet price (the keyword is "Internet" because then they really know that you know how to research actual vehicle price structures, not just fiddle around and send email), have your own financing. Then take your quotes and see which dealers are willing to play poker and outdeal each other.
6. Some people like to haggle for the sake of stroking their ego, saving that extra $147.39, or just prolonging the symptoms of the obsessive-compulsive disease called car-shopping. Hey, I like to save money too, and I'm on a budget, but it may be worth it to you to settle for a SLIGHTLY higher price than what others claim they got simply because you've established a professional rapport with a salesperon whose organized, itemizes and explains costs, and does his or her best to make the whole process as transparently painless as possible. For example, in THEORY I probably could have saved a few hundred bucks if I'd been super aggressive in getting more offers, but my dealer gave me a really good deal on my trade-in, my new truck's numbers were within what I'd expected to pay, and I really liked the way I was treated by the Internet salesman.
Good luck!