Just a couple of points to add here:
a) be aware that generally the thicker the acoustic foam or tiles, the lower frequencies it will absorb.
b) Dense acoustic foam is far more effective than standard foam
c) Rigid fibreglass such as used in duct lining is very effective
d) Corners build up bass frequencies so placing your thickest absorbers there can help
e) ordinary egg cartons (as opposed to the egg carton shaped acoustic foam) are absolutely useless except for absorbing very high frequencies.
f) avoid totally square enclosures with parrallel walls if at all possible
