There is an art to buying a car, but you need to know the important facts.
Firstly, do not go shopping on weekends. The best time to go is during the middle of the week, usually in the 3rd week of the month, and about 1/2 hour before the dealer closes. Why? Well, the best salesmen work on weekends and the newest/worst salesmen work the late shift on weekdays. Since these lesser salesmen don't sell as many cars, they are very eager when a real buyer is ready to give them a sale. Also, the sales manager is willing to give a customer a little more off the price to help make a sale for his salesman. If you go 1/2 hour before they close, you WILL be staying after closing hours while you negotiate the deal. Nobody likes staying a couple hours later than they have to, only to loose the sale because they were inflexable.
Next, the invoice is pretty irrelevant. It doesn't take into account many of the bargains and incentives given to the dealership. In fact, the dealership doesn't pay for the vehicle in full when it is shipped. It is financed by the Manufacturer, and the quicker they can sell the vehicle, the more profit they can make, both on incentives and lowered rates on the manufacturers loans.
There was a site I saw when I was ready to buy my last vehicle. It basically gave you the "buy" price, and it incorporated all the incentives and discounts given to the dealer. It was a fair offer for both you and the dealer as it would still give him a profit, and keep you from spending too much.
Finally, never give them a price you are willing to pay. Make them work at it. When they start the negotiations, send them back to their manager a couple times, saying that their offer is way to high to even begin a serious negotiation. Make them give you the famous "what will it take to get you to buy the car..." line. Then give them a fair offer, considering the discounts and incentives that are hidden from you. Stick to your guns and don't deviate from that price. If it is profitable for them, they will eventually give it to you.
I've done it this way several times. I always get between $4,000 to $6,000 below sticker. My last two vehicles came fully loaded, and I received a written offer from the manager that if I buy the same vehicle model from them in the next 5 years, they will give it to me for the price I paid this year.