When I bought my GT 2 1/2 years ago (ie: when Ford dealers didn't have a problem moving their Mustang inventory), I was able to get it for dealer invoice price (which at the time was $25,560). I basically kept looking at fordvehicles.com, looking for inventory in my metro area, waited until the exact car I wanted was on a lot, and clicked through to the "request a quote" step on the website. They sent me an e-mail with their proposed price (which, from my internet research, I realized was dealer invoice) and I set up an appointment to come in and take it away.
Couldn't have been easier (well, except the whole "waiting until the exact model I wanted was in stock" part).
So couple pieces of advice:
1) do your research. Know what the dealer invoice is, as well as MSRP
2) you're already on the internet - send e-mails to different dealers (see if you can get their "fleet department") asking what they'd charge you for the car you want. Only visit those dealerships that give you a decent e-mail quote.
3) be prepared to walk away if you don't get the deal you want.
2006 GT Coupe Premium, Manual, Tungsten Grey, IUP, Active Anti-Theft, Wheel Locks
Mods: Sequential Taillights, 8" Shorty Antenna, FORD on hood, Hurst shifter with t-handle, Steeda UDP