You didn't state your goals. Are you looking to increase isolation, to remove flutter echo, tighten bass response, etc. Once you know what you're looking to do, the following sites may be of some help:
www.auralex.com - check out their site, as well as their acoustics 101, and acoustics university
www.soundcontrolroom.com - this is the firm that is helping design my new rooms, they also sell products and have good free advice on their site.
You mean an actual dummy wall instead of those half-assed tube-shaped corner bass traps?
To the thread starter... yeah, as has been said, it depends on what sonic deficiencies exist within the room.
As a general rule of thumb however you don't want parallel walls in the control room, as you want to maximize the randomness of reflections. If you're after the Live End - Dead End setup then deaden the front of the room with all the foam you can muster and have a diffusion board behind the mixing position to help scatter those reflections a little bit. You'll probably want to find out if you're getting any standing waves in the room and then get some bass traps to target those particular frequencies... if I'm not mistaken the highs will get taken care of by foam anyway. It's bass that can be problematic... especially in small rooms.