I had to take a couple of courses on studio treatment & design... I never really thought I'd use it, though, as I'd probably just hire someone else to do my professional mixing.
Anyway, putting up rugs, curtains, and, yes, even carpeting, on the wall will help deaden higher frequencies, but won't be thick enough to absorb lower-end bass soundwaves -- the couches are a good start to that, plus it gives you a seat further back from the mixing desk, to get a different perspective. (Or if you have guests over.)
I don't know how much vocal recording you do, but building a small vocal booth in the back corner of the room would a) give you a recording booth (no shit), and b) help make your room asymmetrical and, if you leave the door open, acting as a nice bass trap.
As for the ceiling, personally I'd go for something like this because it a) can act as a bass trap, b) randomly reflects soundwaves, and c) looks cool:
http://www.gearslutz.com/board/attachmen ts/bass-traps-acoustic-panels-foam-etc/1 14190d1237157380-diffusors-small-booth-c ontrol-room-backyard-studio-panels-view-
Pretty much just... avoid having too much that can reflect soundwaves right back in your direction and you'll be fine :)