Ideally, you want hard smooth reflective surfaces opposite absorbent surfaces, angled walls to help break up standing waves (or diffusers if its not practical), aborbent panels behind the monitors. I did some shit on acoustics as part of my degree last year, but f**k me....I felt like I was doing a physics course by the end of it.
*Found this useful...
http://www.soundonsound.com/sos/dec0.../acoustics.htm