TrueCar.com (don't forget to edit options & incentives, then results are in the details tab) - go in armed with invoice pricing and don't look like you've never done it before.
Another way to do it is after you've decided EXACTLY what you want (go into the dealer without talking numbers), call the dealers' internet sales department. Usually these are the folks who know you've got invoice pricing in hand, shopped around, and will give you no BS pricing. "We can do X amount over invoice." Call multiple dealers, quick and easy... just show up to pick up the car.