In reply to dhwsa: Build up slowly, I do 3 sets of 3, holding each rep for between 3 - 8 seconds depending on how much it hurts. I also back this up with fit ball work. Basically do a plank like you might in a fitness class but on a fit ball (physio, swiss ball) then gently roll the ball away from you.
Leg raises can also put a lot of strain on your back, its usually best to do them from a lock of to reduce this and do some small back raises to help equalise those muscles.