Room treatment is, to me, a very important part of good sound. I'd give it a 33% weighting, 33% speakers, 33% other gear. I have seen more than one system at an audio show get best sound simply because the exhibitor was one of the few who bothered to treat the room. And I've done it myself.
Start by treating the first reflections. Put a rug in front of each speaker. Hang something acoustically absorbent on each side wall, where you would see a reflection of the speakers in a mirror from your listening position. Rugs, tapestries, ceiling tiles all help. Then do the same thing on the front wall, between the speakers. If you want to get serious, do the same with the ceiling. In general it's a good thing to treat the front wall all over, not just behind the speakers, because it will help to reduce some of the room resonances. Obviously WAF will be critical regarding this stuff, so consult the SO about what you use for treatments.
Once you have dealt with first reflections you can start to generally treat to room for smooth bass. The two tricks here are to 1) get some free room measurement software and measure the response while you move the speakers around a bit, looking for the smoothest response. Luckily, putting the speakers right up near the front wall can actually work quite well with some speakers. And 2) get some more absorptive material into the room. A soft chair or couch, drapes, wall to wall carpet, all will help. Last, and to me least important, you can put something like books shelves or open cabinets in the back of the room to break up the standing waves a bit.
