I would advise seeing a good osteopath and/or physio to find out exactly where the problem lies as the muscular-skeletal complexities for the back shouldn't be guessed at and there are some exercises that could actually be bad for it. I personally have weaknesses with my sacral-illiac joint for example (the join between the base of the spine and the pelvis) and I also have a slightly rotated pelvis on the other side to this which leads to lower back stiffness and tight glute muscles in my butt! I have some stretches that I use for the glute stuff but I have to be careful with the sacral-illiac stuff as it was actually dorsal raises that caused it to spasm quite recently because I was doing them too fast and not holding my abdominals enough (I think). An osteopath should be able to tell you where the root of your discomfort is and which muscles you should target for stretching and strengthening, which should be a lot more effective than just a general back strengthing exercise...but that's just MY opinion!!
p.s. try to find a reputable osteopath through a personal reccommendation from someone if you can - if you live in North London I can reccommend one myself!
