My advice is to set up your audio gear, then measure the in-room frequency response of your room in your primary listening position. Without any acoustic treatments, you will likely see severe dips and peaks in your frequency sweep. These dips and peaks are caused by speaker boundary interference (having speakers placed too close to walls, typically), room modes, and in the higher frequencies, comb filtering. Go measure your room and report back. You'll see what I mean. The dips and valleys can be in excess of 30db! Seriously go check it out. You'd be surprised what you're missing in your recordings!
Then, before you add *any* treatments, start moving your speakers and listening position (if flexible) around in the room and re-measuring each time. As you do this, your goal is to achieve the flattest response you can by mitigating SBIR and Modal issues at the listening position. For starters, focus on the lower frequencies, below 150hz. If you don't know what I'm takling about now, you will know within 20 minutes of starting this process, because it it will be fairly easy for you to see the frequency impact of the different changes you are making, which are due to speakers being too close to a wall, room modes, etc.
For a long time I avoided measuring my own room because I thought it would be too much of a pain. If you have a laptop, it's VERY EASY. And I have have made more real progress in improving the sound quality of my room in the last few months as a result than I made in the previous two years combined. Seriously, go do it.
Once you've optimized your seating and speaker positions, start adding acoustic treatments, a big batch at a time, and then re-measure your response and see where you're at. Typically speaking, in a dedicated HT environment running multi-channel, you'll want to do the following:
1) Completely deaden the front wall, typically with 1 inch of OC703 across the entire wall and floor to ceiling superchunk basstraps in the corners.
2) 2 or 4 inch panels placed at first reflection points (again, the depth of panel depends upon the room, but there is a school of thought out there -- oft debated -- that says that it's important to use 4 inch panels there if you can get away with it so as to avoid attenuating reflections unnecessarily.) But 2 inch is better than nothing, certainly.
3) Floor to ceiling superchunk basstraps in the rear corners of the room.
4) The icing on the cake may be diffusion in the rear of the room, but there is some debate about how far away from the diffusers you need to be to experience a benefit. I'll have something to report on this soon since I am currently in the process of building a full array of QRD diffusers for the back wall of my home theater.