Extra furnishings and items found in the home can be pressed into service to help absorb or reduce sound. An empty room sounds empty because waves bounce freely. Adding furniture helps reduce this and lowers the resonant frequency of the room, but typically it does this only up to about three feet off the floor. Carefully placed bookshelves can reduce reflections for a fixed spot, like a mixing location. Quilts and duvets can be hung decoratively while also serving the function of reducing high-frequency reflections on bare walls.
One great way to prevent acoustic coupling, which transmits vibrations through the structure of the house, is to place studio monitors on stands. Inexpensive stands can be made by using ABS or PVC pipe cut to length with toilet flanges attached to each end. Fill the pipe with sand and cap with one foot square boards for a base and a speaker shelf. The weight and consistency of the sand will reduce low frequencies transmitted to the floor.
It takes mass and/or large amounts of space to make a true bass trap. You can reduce the amount of bass energy transmitted to the room by mounting two frames four to six inches deep and about twice the area of the face of your monitor on the wall behind each monitor. Fill them with high-density mineral insulation (green rather than pink) and use an open-weave fabric like burlap to cover the insulation.
Often reflections from the ceiling are forgotten. If you have a dropped ceiling with acoustic tile, you're in good shape. Otherwise, consider adding absorptive material there, as well. A box like the speaker traps described above about four feet square over the mixing position will noticeably improve the sound. Even suspending a couple acoustic tiles will perform better than nothing at all.
Windows are reflective of higher frequencies and absorb or allow lower frequencies to pass. In most cases, drapes are preferable to make the entire area absorptive. Adding a small quilt or hanging to the back of a door can help with reflections and sound transmission. It is not recommended that all surfaces be treated to absorb reflections, as this will lend an artificial effect to the room's sound.