Back problems

How is your position on the bike?
Are you rotating your hips forward to give a flatter back?
Hamstring stretches are very good. Tight hammies (which you get from cycling and overdeveloped quads) drag your pelvis down which in turn drags your lower back down = pain. Hamstring stretches rule and should be done anyway but check your position as well.

Cyclefit can be helpful if you have a spare £200 :)

Ive been a massage therapist for the last ten years and I can say that, Yes, Hamstring stretches and quad stretches on a regular basis are extremely important! Yoga and pilates are an amazing thing too if you take them seriously. Forget any hippy crap connected with it, It is a legitimate stretching and strengthening regime that will benefit you massively through any back problems you might be experiencing. Doing Yoga (Ashtanga) changed the way I do everything with my body. riding especially! Check your seat hight too.