How can I alleviate a sore back?

Depends on reason. So many causes of sore back. If it's a long standing low back issue then laying on your back on the floor with your legs on sofa helps with small movement of bringing your tail bone to your belly button works. A great physical therapist working on your core is best with some ibuprofen. Yoga/pilates is a great starting point. If you have pain that radiates or incapacitates you, see your doc.