I just do some research
I go online, do some research about the market value of the car I want to buy, get a reasonably objective assessment of the trade-in value of my car and try to drive toward that with a dealer. I find that I'm usually at or very close to those 2 numbers and I consider that a fair deal - not great for me, but not something where i'm leaving alot of money on the table for the dealer, either. Sometimes, I do better and sometimes not - it mostly depends on the popularity of the car I'm seeking.
That's really only the first battle, though - the "Finance" guy is now driving lots of hard bargains to boot, what with trying to sell add-on warranties, financing, dealer's fees for registering the car, etc. I just disallow as much of that as I can, sign a waiver that says I'm responsible for registration and any applicable taxes, then go work it out with the MVD. It's usually hundreds cheaper that way.
-- mike