Tim,

Depends on what you want to buy.
Dealers are clearing out NEW 06 Mustang GT's at just shy of 8,000 off MSRP
(that is under invoice price).
(I've posted scans of the newspaper adds over at
alt.binaries.pictures.drag-racing a little while ago).
If I were buying an 07 Mustang GT, I would be paying INVOICE price or less
for it.

Don't waste your time talking to the sales drones, speak directly to the
sales manager.  If you don't know who that is, you can call and ask his
name, or stop in the parts department on your way in and ask the parts guy
the sales/general manager's name.
Then when you go into the show room you can ask for the sales manager by
name.  Tell him exactly what you want and how much you are paying, ask him
if he would like to sell it to you or not.
If you don't know exactly what you want' don't make an offer.  If you ask
questions and are indecisive, they will OWN you, you will be their bitch.
If he tries to play games walk out.

YOUR NEGOTIATIONS SHOULD ALWAYS START AT INVOICE PRICE!

I have done this on my last three new purchases.  The first I paid invoice
price (done deal over the phone with the sales manager, and got high
wholesale on my trade-in too), the second $1800 under invoice, and the last
(2005) $3000 under invoice.  Don't take any BS about not getting the
incentives too, you always get all available incentives, no matter how good
of a price you get, or the dealership keeps the incentives themselves.

DO NOT FALL FOR THE EXTENDED WARRANTY, UNDERCOATING OR PAINT TREATMENT
SCAMS!  These items are sold at many, many times the dealers cost.  Premium
extended warranties cost the dealers well under $1000.  The only one I
bought was $100 over the dealers cost, I paid $500 for their highest level
extended warranty.  They will sell them for as much as they can get.  Dealer
supplied undercoating and paint treatments are simply a rip-off aimed at the
unfortunate misinformed.

I am now on first name basis with the dealership owner of the Ford
dealership I bought my 2005 ($3000 under invoice) at and have ordered my
Shelby GT500 Mustang from him (that's a whole different market).  He knows I
buy new cars, I know he sells cars for a living and he doesn't screw around
with me.  We both get a good deal, it works out great for both of us.

Basically car salesmen will treat you like you ask to be treated, if they
think you are a fool, they will treat you that way.

Nobody