As a former car salesman, here's my 2 cents.
1st - know exactly what you want, color, options, etc.
2nd - know the MSRP and the dealer's cost (others have advised you on where to find that info)
3rd - demonstrate your ability to buy and your sincerity. Do this by getting pre-approved on a loan and be willing to leave a reasonable deposit, say $250.00.
4th - make a reasonable offer, $1,000 over dealer cost, $500.00 over dealer cost - whatever you believe to be reasonable - and advise the dealer of who has approved your loan and a person he can talk to about your loan.
5th - Make your offer and stick to it!!! If you can't walk out, you can't get a deal.
If the dealer will not meet your price, then shop at another dealer. Tell the dealer that you appreciate his time, but you will not pay more than your offer, so you will need to visit a few more dealers in the area. This puts the dealer in a position of watching a sure sale walk off the lot. Many will let you walk, but sooner or later a dealer will meet your price.

I ordered my 05GT in November, I went through five dealers within 50 miles of home, prices ranged from $3000 over MSRP to full MSRP, finally made a deal for $850 over dealer cost.

Also, check out each dealers "document fees" and other delivery fees, some are very high, my dealer only had a $50.00 doc fee. Stay away from dealer arranged financing, you will get your best rates by finding your own lender.

Hope this helps.

John