In my little room I'm using a combination of Helmholtz Resonators and Panel Absorbers... here are some
formulas.
For the Helmholtz you have to find out the exact frequency modems to use them effective, the Panel Absorbers are working a bit broader but you still should analyse your roomsituation correctly. If you are handy in calculation and woodworking you might do it yourself. I did the calculations and planning by myself, though I had help from a woodworker, it saved alot of money at the end.
good luck... Andreas