You should first click here to get a price quote here for a new Mercedes. Now you have some real basis for what real deals are to be had.
Search online for user reviews on Mercedes Auto Dealers in your area.
Find a post where someone else has done the work negotiating work and put the specifics in their post.
Print out the details with their dealerships name on it and carry it into the dealership requesting the same deal.
Some of the large metro area dealerships are artificially high.
With that in mind you might want to go to a smaller nearby market and do most of the negotiating on the phone.
Look and call around. There are only so many Mercedes Motors Dealerships in Texas.