One of the biggest things people can do is to move the speakers off the back wall. The more the better. The back wall being the back of the soundstage, which some confuse in calling the front wall.
If you can get a 8 millisecond delay from the back wall reflections you will enjoy a much cleaner, direct presentation. You will also get a fantastic soundstage. One may give up low end thump and drive however, depending on your speakers, unless you have some kind of e.q.
Another biggy for me is to get your speakers apart. So many are content to listen to a t.v. sized musical presentation. I want BIG, life sized sound.
In my system I have my speakers 12' off the back wall, well sort of - I have some side walls coming in closer, but if I could I would rip them out I would. The speakers are about 10' apart, and about 9-10' from me. I usually prefer the speakers towed out about 10-15 degrees from directly pointing at me, on axis. This gives more air, and is more relaxing than on axis, or towed in.
I think the ideal room is about 16-18 feet wide, and around 26' long, with a sloped ceiling. This is the BIGGEST improvement one could make in a system.
I always get a chuckle when people talk about the fantastic improvements a powercord, or component, makes to the system, when the system is shoe horned into a puny untreated room. WHO CARES !!! The system is lame without the room. It may sound good, but it will not blow your mind with a 3d, holographic, real, presentation.
Just my very humble, unbiased opinion of course.

Rocket_Your room is EVERYTHING_Ronny