Having worked with acoustics a bit, I can only echo the egg cartons and drapes for a cheap solution. Use drapes and carpeting where cartons not practical (drapes especially can be found cheap at goodwill stores and auction marts). Drape strips hanging from the ceiling help.
Isolate all sound equipment with sound-absorbent material. I found a box of cork wallboards, about 1 1/2 inches thick, and put two separated by small blocks under the equipment. (Also good for walls, but expensive new- try demolition sources).
In the other thread, Spike has a point about "live" sound that I can't answer to. I have worked only to deaden sound and to kill bounced sound.