Absorption or diffusion????

I think either would do the trick. But absorbtion is much easier and cheaper.

A quote from a smarter guy then me

"For someone with a very small budget, making the rear wall of a room totally dead may be the only solution. At least that gets rid of flutter echoes between the front and rear wall, though at the expense of sounding stuffy and unnatural. But it's better than the hollow boxy sound you get from a plain flat reflective surface. Another option is to make the rear wall of a room partially reflective and partially absorbent. You can do this by making the wall totally dead, and then covering it with thin vertical strips of wood to reflect some of the sound back into the room. If you vary the spacing from strip to strip a little, you'll reduce the coherence of the reflections a little which further improves the sound." - Ethan Winer

Either Absorption or Difussion should work. But I like Shiva's idea the best.