Rest, heat, massage. And use of over the counter medication if no medical reasons for not using them can help as does time to heal and avoiding activities that may provoke the symptoms. If they persist or worsen, see your doctor especially if associated with any fever, chills, sweats, appetite loss, weight loss, fatigue or generalized malaise.
Stretch & strengthen. Back muscles spasm when they are asked to do something that are not capable of doing resulting in a strain of the muscles. There are many treatments available, from medications, manipulative therapy, physical therapy and home exercises. I generally recommend daily stretching and strengthening exercises. See my back pain/back strain healthtip under "superman" exercises.