I used to experience lower back strain myself. All very good suggestions listed above. This helped me:
1. Play with the guitar on your right leg if you can
2. A little foot stool for your right let will help also so that you dont slouch
3. A comfortable chair that allows for good posture. Putting a pillow on the chair to support your back (particularly lumbar area) will help great too.
4. The other thing for me was playing relaxed. If your fingers are tense trying to hit the chords correctly, then the rest of your body (particularly your back and neck) tend to tense up also. A good 5-10 minute warmup to get the fingers loose helped me in this area.
5. Practice for 15 or 20 minutes and take a 5 minute break. You can also alternate practicing sitting down and standing up.
The best exercise for immediate relief if your feeling pain in your back is to lay flat on your back (on a hard surface like the floor) and pull your knees to your chest. You can also just lay flat and make sure your legs are elevated...whatever pain you had will have immediate relief.
Back pain sucks especially when your trying to do something you really enjoy. Hope this helps you.
TCR