If it is really severe, the typical advice applies: go see a doctor.
Otherwise, the following usually helps me with back strains:
1. Rest (duh).
2. Massage, myofascial release (a tennis or lacrosse ball can be really helpful) and/or a visit to the chiro (especially for ART).
3. Really light weight, high rep exercises. The goal here is to flush lots of blood through the back muscles, and NOT to challenge the muscles in anyway. A few sets of ~30 reps of supermans, back extensions, good mornings (with PVC or a broom stick), cat stretch or hip bridges can really warm up the muscles and make everything feel better. You should feel a pump/burn and maybe some slight discomfort, but if it hurts, stop.
4.
After the muscles are thoroughly warmed up, some stretching might help. You typically aren't trying to stretch anything in the back itself, but rather surrounding muscles that might be tight and putting pressure on it. For me, stretching the hamstrings (favorite: holding a deep good morning with the back straight) and the glutes (favorite:
prone glute stretch) can often help the back relax.