I once had my system set up in an 11 x 12 room and ended up fitting my big Altecs in there (!), so developed a bit of a methodology on getting something reasonable.
I started by getting everything out of there that wasn't necessary and it turned out, that was a lot. I got down to the stereo, a coffee table, end-table (that doubled as some record storage), a lamp, clock radio and a pull-out futon (yes, it was also a bedroom) that's it. The cloths I used regularly all went into the closet on a shelving system, anything occasionally used and all the computer stuff went into another room only a laptop remained.
I listened and messed a lot with system and speaker placement (Coincident Triumph sigs on stands) and it was better, but still pretty bad. Luckily, the main problem was a room that was way too ‘live' (drywall, 2 windows, 1 door, a huge ‘mirror' faced closet and hardwood floor) so I put in an area run and considered my move to absorption panels targeted at mid into high frequencies. Pretty close to a square room which is bad for bass node problems but I was renting so I'd just have to live with that.
I lucked into some old office partition panels that they were replacing at work and these ended up being the best ‘system' component I'd ever added. They were real heavy (HDF core) with rock-wool on both surfaces, burlap wrapped and decent looking oak edges, even had plastic ‘ footers' that allowed them to slide very easily in front of that class faced closet and one of the windows when really listening.
The sound in there improved dramatically with these panels and I had enough of them that I experimented with several until I found a nice balance. The final piece that was really a shocker was a speaker upgrade those big-ass 9 cu.ft. Altecs stuffed in the corners actually gave me more usable space in that room than those monitors on stands and I feel the point-source design worked considerable better when sitting that close to them in there.
So it was purge the stuff, less-is-more, get a feel for the room straight-up, carefully tune to taste, then experiment with tweaking the system as icing on the cake even something as dramatic as a completely different type of speaker. There is certainly a more scientific way to do this and it's probably more effective but hack and taste worked pretty well - considering the limitations of a 'temporary' space.