Everyone else is right. Never, ever pay more than invoice for a car. It's a buyer's market, so you have the upper hand. Do you homework prior to going in to the dealer. Know the invoice price (find it on Edmunds, Kelly Blue Book, etc.) And also have a pre-approved loan. That way, you can decide whether or not to take dealer financing. That's more leverage for you, because you know exactly how much you're planning to spend, and also because the dealer makes a commission on their financing also.
The biggest thing is don't get emotionally wrapped up in the prospective new car. If you do, the dealer will smell it like sharks after blood in the water, and you'll have lost. If you they won't meet your price, get up and walk out and go to the next dealer.
Also, wait until the end of the month, go in on the last, or next to last day, about 3-4 hours or so prior to the dealer closing and take your time. They're going to want to make a sale at the end of the month, and they'll stay as long as it takes to do so. I once kept a dealer 2 hours after they closed negotiating, and I got my price. It's a mind game, a battle of wills, and you've got to be stronger.