When building up the first incarnation of Rocinante Studios in San Francisco before moving to LA, I read up on acoustic treatment. Initially I went for the basic approach of foam panels, and got a bunch of cheap ones like these from Amazon. Auralex is usually the company that you'll hear about, but I didn't want to spend much cash at the time, and these were cheap and made a difference. However, they won't do much for the low end, and only really stop reflections above 400-600Hz, which means you're screwed for proper bass handling. For that you need bass traps, which you can get foam versions of. I just didn't bother at the time. Still, a start's a start. Excellent article here, and accompanying image:
I did manage to get some great recordings in the room though, but this was mainly for vocals. Acoustic guitars recorded with mics had a definite boxy sound. I could still work with it though with some friendly EQ. I used an acoustic shield for vocals which really worked wonders at a low cost. You can see my friend Alex singing with one in the image below, and here's a link to the one I used. Might not be the fanciest on in the world, but it did a great job at giving me a dry vocal in a non excellent space.
When I was building up the new studio in LA, I initially used the same smelly foam, but with the intention of getting professional acoustic treatment. I used the 'sound on sound' diagram for reference, and covered all the main reflection points with the 2" foam. The sound was actually ok, but probably not great bass handling. As mentioned, I did notice a smell coming from the foam, which my wife wasn't too happy about.
After some initial research I found a company called LA Sound Panels. They charged a $50 consultation which was deducted from the overall price if I a purchase was made. Installation was extremely quick, and I was super happy. Also, you can choose the colors which is great.
After this, there was a huge difference in sound. The bass became a lot punchier, but never overpowering, and I could hear a lot more happening in a mix than before. One weird thing that happened though was the fault of using two computer screens. Here is my initial set up, where I have screens connected to a Mac Laptop. I started to notice an issue with my left ear, which I'd noticed before, but not to this extent. What was happening was that my DAW (Logic) was always on the screen on the right, which meant my head was always tilted that way, in turn exposing my left ear to more sound from the speaker than my right. This would very quickly fatigue my left ear and leave me feeling dizzy. It took a while to work out what was happening, and then I felt stupid for not working it out earlier.
To remedy the problem, I position on monitor directly in the middle, which has helped a lot. I also upgraded my monitors to Yamaha HS8s which have made a big difference too.