Hey
I meant to say in my last post, what's been discussed so far is really acoustics, i.e. shaping the sound environment for recording, rather than sound proofing, ie.e stopping sound escaping from your studio so that you can a) reduce external sounds spilling onto your recording, Keep your music spilling out of your studio.
Back to your standing wave problem. Prometheus, what sort of frequencies are you having standing wave problems? There are several obvious changes you can make that are more focused on addressing the problem:
Make the walls not parallel
Use a bass trap focused on key frequencies
Both!
Using acoustic tiles, egg cartons etc can be effective at dealing with high frequencies, but they do almost nothing for low frequencies.
For the parallel walls, the easiest is to put a thick strap of wood vertically down the middle of the wall. A thin strap on either side. For simplicity make this the distance of the width of plaster board. Nail the board to the strapping, creating a "V" shape on the wall. Fill the space between with rock wool. Cover the surface with fabric.
It's pretty cheap and cheerful, but it can also be easily removed if you move your studio.
I wrote a post somewhere on here about building a bass trap. I'll try and dig it out...
Cheers
John