There was a great post on the old forum that had a link to graph that showed what wire gauge you should use for a specified distance. I think Woochifer or Smokey posted it. It was pretty cool as it accounted for any signal degradation over distance. You might want to check that out.

As for the question, obviously if you can get into your attic or crawl space and drop some wire between the sheetrock, that will be the cleanest install. Put some nice Niles faceplates on it and, viola'! You're good to go.

If that doesn't work, using good quality flat wire such as Nordost Valhalla (NOTE: I didn't say use Valhalla, I said LIKE Valhalla) will make tucking it under the pad much easier. If you don't have carpeting in your room you could easily get some pre-fab crown molding and tuck it into the ceiling/wall crevice and conceal it behind the molding (in this case you could use normal round wire). This is perfect for ceiling or high wall mounted installs and you get the added bonus of dressing up your HT room. Crown moldings are always classy.

Good luck