First off, the back is part of your core, so i find it hard to beleive you have a strong core wiothout a strong back? At lead the lower back anyways. COre movements that would benifit you are more the pilates type, working on contracting the entirety of the core to support your back.
Secondly, do the back hip stretches and go read the article that was just written on stretching and mobilizing the hips.....
Lastly, train the fuck out of your back, core, legs. USe movements that decrease or elimate compressive forces like split squats, cable rows, supported db rows, pulldowns, glute ham raises, lunges, one leg legpresses.
Stay away from squats, deadlifts, bent over rows, other things where you bend over and have to hold weight, also NO FUCKING SITUPS!
if you want i can get more in detail about the core movements and other back pain causes as ive delt with the whole shabang.