Bought a car a few years ago, and found this non-profit that had a great strategy:
http://www.checkbook.org/auto/CarBargains_Secrets.pdf
You can never know what the dealership is getting from the factory in terms of kickback, so it's next to impossible to negotiate a deal all by yourself. The sales rep is never going to lose money on the car (despite what they may tell you); they'll just walk away. So even when they cry and tell you you're keeping them from feeding their family just know that they're making enough to cover their expenses. Your best bet is to put your purchase out to bid to multiple dealerships and let them fight it out. We did this and saved $2500 off the "invoice" price that Consumer Reports said we should be "aiming for" to get a good deal.
Let me say it again: make them bid; it's the only way to keep them honest.
As a side bonus, you don't have to deal with crazy add-ons, haggling, or waiting for managers to "approve" your deal. You e-mail the dealerships, tell them what you want, and ask for their final, out-the-door, all-fees-included price. Pick the winning one, print out the e-mail so you have it in writing, and go to the dealership to pick up your car. If they try to add anything on, just point to your e-mail and invite them to throw it in for the included price you've already committed to (we got "free" floor mats and locking wheel nuts, probably because they didn't want to bother to take them off).
Note that you have to be willing to contact multiple dealers, wait for responses, and follow through. If you want to buy the brand-new 2014 model whatever, in hot pink, and you need it TODAY, then this isn't the strategy for you. If you're willing to be patient to save a couple grand, try it out.