difussor's

In small squarish rooms, many have had success setting it up across a corner rather than a traditional setup. If you are free to give it a try, you've got nothing to lose. 

Generally I would agree that absorptive bass traps in corners and higher frequency absorption at first reflection point is your best bet for starters. A little absorption at center front wall between speakers can also help tighten up your imaging/soundstaging. Cheers,
Spencer

Definitely diffusors behind you to, well, diffuse back wall reflections. I used two Aurelex diffusors in my former listening space and these really made the back wall recede.  And absorption of first reflections to the side walls is a must to reduce phase cancellations.  A couple of floor-corner bass traps won't hurt either.

I built five 2 x 4 foot quadratic diffusors for my front wall. I bought the plan from Acoustic Fields. Check out their website they offer a lot of good information. They really added depth, width and clarity to the soundstage. It was a lot of work but well worth it.

Diffusors (at least 1D diffusors) will not deaden the sound in the room, but can clear up and stabilize imaging while leaving liveliness in. If you have dipoles, then diffusors behind the speakers is probably a good idea. Otherwise, they usually work best behind or to the sides (but should be at least 4 or 5 feet away from your listening seat).

If you don’t mind a little (actually very little) work, and are in the USA near some lumber or big box hardware stores, you might want to try my DIY 2D diffusor design (actually designed via computer simulations rather than math formula or cut’n’try). See: http://www.diyaudio.com/forums/everything-else/269366-making-easy-diy-depot-sound-diffuser-panels-st...

If you are in a metric country (and don’t mind a little more woodwork), take a look at Tim Perry’s "Lean Fusers" at: http://arqen.com/sound-diffusers/