Ok, big disclaimers:
I'm NOT an expert. Just a dabbler. I have not even treated a single room (although I'm in the middle of doing mine).
My recommendation: go to the "Acoustical Treatement Master Thread"http://www.avsforum.com/avs-vb/showthread.php?t=255432
It's a huge thread, but if you take your time and read it, you'll get the general idea of what people are doing. If you really care about sound quality, you'll then likely hire Bryan Pape, Terry Montlick, Dennis Erskine, or another professional to do some online consulting. It doesn't cost as much as you might expect.
If you don't want to do that, then you can muddle through and do some basics. Here is a summary I just put together for a friend:
1) get absorption on the first reflection points on the floors and walls. This is the spot on the left/right wall where the sound from various front speakers bounces off the wall and hits the listener. The floor is usually handled by carpet, and the ceiling can be done as well, but most people don't for hassle/aesthetic reasons.
With three front speakers and multiple people in the room, it ends up being a range of dots on the side walls. Put carpet on the floor, and put 1" thick fiberglass or cotton panels on the walls to cover those spots. Lots of advice in the thread above on how to frame/cover the panels so they look ok.
The gain from this is better 'localization'. The soundstage of the front speakers sounds accurate (right sounds from the right, middle from the middle, etc.) and not mushed together.
2) cut down on the echo in the room. This is generally referred to as getting the rt60 under control. (RT60 is ~ the time it takes for the sum of all reflections to decline 60dB once the sound source ceases). This is done by covering a certain amount of the walls (often ~50%) with material that will absorb mid-high frequencies. 1" acoustic material is typical. The first reflection point (FRP) absorption above counts towards this coverage. Often, there isn't any special FRP absorption: people just put absorption on the side walls up to and a bit above ear height along the whole side wall. This takes care of 1 and helps with 2.
3) put absorption on the front wall. Pretty much make the front wall dead (2" fiberglass panels). Same with the back wall. The front wall keeps all the surrounds from bouncing back at you. The back wall absorption (typically close to the back of the seats) counts as a 'first reflection point'. I'm doing 2" fiberglass panels across both front and back. (Some folks prefer to keep the back wall reflecting. "Live-end/Dead-end" was apparently a common desire in music rooms. In my HT, with seats 3' from the back wall, I want the back wall dead)
4) Bass traps. Typical room is going to have resonant frequencies (wall to wall, ceiling to floor, diagonal corners, getting more and more complex). At these frequencies you will get standing waves in the room. Those waves will tend to lead to spots in the room where certain frequencies of bass are much stronger than they should be (peaks) and spots where a certain frequency is completely non-existent (nulls). This is bad. At any given spot in the room, instead of a flattish bass frequency response, you'll get these (potentially big) peaks and nulls. This is typically in the 20Hz to 200Hz range.
The problem with these standing waves is that you can't equalize them out if you have more than one listening spot, and maybe not even then. In your seat, you may have a null at a given frequency, but two seats over the same frequency may be a peak. If you raise it up to fix your null, it makes it even worse for the other spot.
Typical thing to do here is to build bass traps in the corners (corners because all standing waves have at least some connection to corners, so you can be most effective there. It's also often the easiest place to get a deep bass trap. A very easy and well reputed DIY trap is the studiotips superchunk (http://forum.studiotips.com/viewtopic.php?t=535
). Triangles of fiberglass panels stacked in the corners. There are also panel bass traps you can buy. In general, more bass traps is better.
So: it's much more complicated than that, but it seems safe to say that if you have:
- carpet of some kind between speakers and front seats.
- front wall dead.
- 1" absorption up to ear level on the side walls.
- Bass traps in the corners
That you will have a better sounding room than if you do nothing at all. Will it sound better (or can you get away with less/different) if you hire a pro? Probably.
Good luck, and remember, this is simply my (imperfect) learnings from reading the master thread. It's worth what you paid for it...