A lot of times in the clinic I am asked by patients, "how do I go home and relax my back when there is discomfort?" And really the key to relaxing your back is going to be to try to encourage the neutral spine or the natural "S" shape of the spine.
There is a natural "S" shape and you want to try and promote it with your positioning. Sitting is hardly ever a good option, it's usually going to be a lying down position and I can argue that on your back, on your side, and on your stomach can all be utilized if you're utilizing it correctly with proper props or proper support structures. For instance, on your back, if you would just bend your knees a little bit, just by bending your knees up a little bit that immediately sort of helps to release some of the tension in the back.
Sometimes patients benefit from sliding a pillow or two underneath the thighs and if you let your legs fall on top of that now so the knees are bent just enough to take the pressure off of the lower back. Some patients would like to slide a little small towel underneath the lower back as well. If you're going to come onto your side, for instance, you would want to maybe put the pillow in between your knees, you've heard maybe that before.
The key with being on your side is going to be to line up everything in neutral so the ear, and the shoulder, and the hip, and the ankle is all lined up in one line and that will also help to relax the back. Outside of your lying options, the other options for home could be to use a warm compress, it seems to soothe the muscles a bit.
Lastly, trying to remain a little active, it's called "active rest", where you're not being too stationary in any one position but also not running around and getting too much done. It's called an "active resting period" and that usually will help to relax the back a bit.