I suggest using hard rockwool. It is a great sound absorber and if it is thick enough it can even handle bass, which usually gives most trouble. 10 - 20 cm should do the trick. It must be wrapped in wool to stop the dangerous rockwool particles reaching your lungs. I think the standard industrial wool is 3 cm thick. Make nice wooden frames and wrap everything in burlap and you get cheap panel absorbers.
Also you can use thin wooden boards combined with above mentioned rockwool absorbers, but each board resonates at some freq. so you need to calculate where it has a peak. Boards need to be away from walls/ceiling and rockwool flattens the peak so you get broad freq response.
Actually if you have a big room, then move the rockwool panels away from the wall as well. It reduces the room, but gives best results.
Also note that too much absorption reduces the overall level of sound in the room.. not to mention that it can sound really strange to suddenly lose the reverberation.
This is the most basic procedure. I suggest you check out this really nice diy design
http://thegilpins.org/Projects/Site/Pro … 8F17F.html
but you can also make them uglier, with less trouble
I built mine without the wooden frame so they look like big pillows and are easy to transport.
good luck