Through the same hole that you are bring the signal wires in, fit a 16" - 18" length of 1/2" piece of wood about 2" wide behind where the spline of the mount will be. Perhaps put a quick drywall screw in from the sheetrock to the wood to hold it temporarily in place.

Now carefully screw in the mount using the alotted holes into the wood behind to hold it. (pre-drill for #12 screw perhaps). The wood will support the bracket between the sheetrock.

I'ver used this technique with 42" plasma screens as well. If done right, it'll work like a charm. Honest

You can thank me later.