Absorption and diffusion in a wall-mounted unit that costs (next to) nothing
There is such a thing as good reflection of sound, and bad reflection of sound. Good reflections are those that are spread out over a wide area, so that at any one listening position there are many reflections, but they are all low in level and none is prominent. A bad reflection on the other hand is one that is strong and one of just a small number of reflections. It will be clearly audible, and in the studio control room will cloud your judgment of how the mix should sound.
When builders build a room, they make it cuboid in shape (a cuboid is a three-dimensional rectangle, like a shoe-box), and they finish it with hard plaster surfaces. Guess what? They have made the worst acoustic environment going. And since all rooms that are not purpose-built for recording start off this way, plainly something has to be done.
One important component in creating good control room acoustics is to give the walls a balance of absorption and reflection, and where there is reflection, try and break them up so you get lots of tiny reflections heading out in different directions. We call this 'diffusion'.
Absorption can be created using porous absorption such as mineral wool (mineral fiber). At low frequencies, membrane or panel absorbers can be used. Diffusion can be created by mounting reflective wood panels on the walls combining a wide variety of angles to spread the sound.
But all that takes time and money. Isn't there a cheaper, quicker way?
Well yes there is, and the answer is to line your walls with books. The books need to be all different sizes, hard cover and soft cover, as then you will get absorption and diffusion across the frequency range, and the bookshelf you stand them on will help with the diffusion too. And books can be acquired for next-to-nothing at a jumble sale/yard sale (or even a car boot - there's a nice piece of UK English for our American friends). Just wait until everyone has gone home and make an offer for what's left.
"What about the bookshelves, surely they will cost money?", you ask. Not if you can build them yourself and get the wood for free. Close to where I live there is a glass merchant. Their glass comes packed in wood, and they have to pay to get rid of the wood. So they offer it free to anyone who wants it and can take it away. It's not the best quality wood in the world, but it's usable. Maybe there is a similar source close to you - it doesn't hurt to ask around.
The other great thing about books, by the way, is that if you get bored after a hard day's tracking, you can read them! You might even get some good lyric ideas...