I have two rear speakers, that I never use, since I don't have a good way (yet) to wire them.
I have a hardwood floor. And the side of the room that is nearest to the TV+Audio source is open. So I don't have a wall to run the cable next to it.
If you're installing in room where you can't drop above or below the room to run your cables, consider putting up crown molding -- it typically leaves a small channel behind it which is ideal for hiding speaker cables.
You can then punch holes in the top of the wall where you need to run vertically, fish the wires through, staple it up into the corner, then install the crown molding to hide everything.
You can also go the low route, and hide them behind your baseboards, but there isn't always a void behind them, so this could get a little messier if you have to trim away drywall (or plaster in older homes) to create a channel you can use. You also have problems with baseboards if you have to go past any doorways.
Have you thought about using in-ceiling speakers? Then you could fish the wires through the ceiling (assuming the joists run the right way or you have access to the attic).
Easy setup + affordable wireless solution are now available. There are a few soundbar systems with a set of wireless subwoofer and rear satellite speakers
For example the Vizio S4251W-B4
I would just get a sound bar.
not as good as actual 2 back speakers, but better then 2, and can't beat its simplicity (one item replacing 4 speakers, a dvd player and the AV box).