Study up. Use KBB.com and edmunds.com to get ideas on how to buy a new car and prices, as well as what your trade might be worth, remember to negotiate the new car and you trade separatly. Those sites will list MSRP and Dealer Invoice as well as what you should pay for said automobile. Remember, you are in control, you are the one buying, you have the cash, you don't like the deal walk out. There are always more dealerships that want to make a deal. That doesn't mean offer 20k on a car that's worth 32k, but by studying up and using the hints and prices you find on line you should know what a good deal is and can bargain accordingly. When you do come up with a deal, then ask about a further discount about paying cash. Depending on how much you can get for a rate to finance, you might be better to finance and put that money away and let it work for you.
I just bought a new 2013 Ford Explorer for $236 below dealer invoice earlier this week. I got it on the last day of the month when dealerships are looking to move cars to make quota for the month. I was dealing with one dealer and called the other and the second beat the price over the phone. I took the $2000 rebate over the 0% financing, but was still able to get a rate of 1.74% from a local credit union with the dealer doing all the work for it. I talked to a fraternity brother who's a finance manager at a different car manufacturer and he said that 1.74% was pretty much a wholesale rate.
When you study up and know what you want to and should be paying it's a great feeling, a lot better then feeling like you were taken.
Good luck