Sparkey beat me to it..
Focus on the primary reflection points like Tim mentions- walls and floor (rug w/under-padding at the reflection point) and possibly ceiling, depending on how far you care to go with it. From what I understand you don't want to cover too much wall with thin-ish absorbers, or the top end gets overly absorbed and the room sounds overly dead, like a badly treated garage band rehearsal space with thin foam everywhere. I hate those spaces. I like a more lively, though controlled room. I've played with putting couch cushions up on the back wall at the reflection points in my room and I don't like it, kills the 'air' and the sense of depth in my room. You might try that just to get a feel for where you're going with it first. You might find just hanging a bunch of stuff on the wall or sticking a book case or something there works better by diffusing the reflections off the wall and breaking up slap echo between facing walls. Diffusion kills slap echos and specular reflections without reducing the reverb time of the high end. There are also combination diffusive/absorbing materials and ready made products.
Its the bass region where you really can't get too much absorption (bass trapping) in a small room. Both because it takes more to do the job and because bass end absorption is big and expensive. Too much thin absorption on the walls can actually work against you by damping the high frequency reverb without effecting the low frequency, exacerbating typical 'small room' bass problems. Solid concrete walls don't absorb much bass. Windows and doors will act as bass traps somewhat, passing the low frequencies and reflecting more mids and highs.
Once I get around to it, I'm thinking of building some nice looking wooden diffusers for my back walls (diagonal room setup) and scheming ways to build some 'dead air behind rock-wool' low frequency absorption into a couple room corners where they'll do the most good.