Good home-repair question — answering as the UphillTech team:
Monoprice has a complete kit that will allow you to cut two holes in the drywall, and then get power and also run your signal cables vertically. I believe you need to install an 'old work' (aka 'retrofit') box for the power side, but that is pretty straight forward. Once you cut the holes, these things fit in, and have wings that tighten down onto the drywall.
Full answer by gregmac: https://diy.stackexchange.com/a/783 (CC BY-SA 3.0).
Need help with this? UphillTech.