Healing my lower back?

I've known how to lift properly for years and have followed it pretty well up until recently. I lifted a 21" CRT monitor at work and hurt my lower back. This was 6-8 weeks ago and it was feeling great up until today, I was hanging out at the park with some friends, we were throwing the frisbee around and such, and after a little bit my back started hurting again. I know some people are going to say "go to the doctor." I have no fear of doctors, I'd just rather not fork out the dough.

So my question is, what exercises and other things can I do to heal my back, I'm not expecting immediate results, but just things I can do for like half-hour a day that would help strengthen my back would be great.

If it makes any difference, I'm 22 and am going to start back karate again very soon for exercise purposes (within a month), and I've done karate previously for 10 years.

-sleep on the floor on a sleeping back--sleep like a plank with your back on the hard floor (this helps A LOT--do this for a week and it'll feel MUCH better)
-do the cat stretch and its opposite (on your fours)
-do the seal stretch
-continue to exercise lightly--like do the elliptical at a moderate pace for 30 minutes
-don't slouch, sit up straight--esp. when in sitting in your chair, stretch your back by arching it (this hurts like a ****** but it helps)

good luck!