The thing that fixed my back was deadlifting. I bought "Power To The
People!" did my best to follow the instructions, suffered a few minor tweaks along the way, but it's been the main thing that works for me and, if I don't do it regularly or something similar, my back starts to bother me more. (The substitute for me is the kettlebell snatch, which is a lot less of a mainstream sort of thing.) Pavel (the author) has a great line in his flexibility book, "Relax Into Stretch", about why you should _not_ try to stretch to relieve back pain. Here it is:
"Spasms and pain are only symptoms. The real problem is usually weakness. A weak back muscle has to contract hard just to keep you from walking on all fours - spinal erectors are 'anti-gravity muscles'. This tension is difficult to maintain, so the muscle just locks up. Movement and circulation become limited, so it gets even weaker, so it cramps even more to get even weaker to cramp even more... It's a vicious cycle.
"Conclusion: trying to fix a bad back with stretching is about as useful as an oil change on the Titanic. You'd better get on a first name basis with deadlifts."
And that's from a stretching book!
I suffered a debilitating back injury about 6-1/2 years ago - couldn't walk for about six months, took over a year until I could walk without a limp. If you want to follow your doctor's advice and learn to strengthen your lower back and abs, deadlift. Be careful, stay tight (PTTP spends a great deal of time talking about what it is to stay tight, why it's important, and how to structure your workouts to maximize its effectiveness), and give yourself a few months to start to see signs of progress - and a few years to see real results that will be worth the wait.
Links to the above-mentioned books below.
-S-
http://www.kbnj.com
http://www.kbnj.com/ptp.htm - Power To The People!
http://www.kbnj.com/ris.htm - Relax Into Stretch