Pregnancy leg cramps are the worst! I have had them in both my pregnancies and they really are a different breed from your normal leg cramp. (ie MUCH worse. I remember thinking during my first pregnancy that they were good practice for coping with labour and in fact they were as even the pain of transition wasn't as intense as some of the cramps I'd had during pregnancy). At my last visit to the Ob (I'm now 34 weeks) I asked him about the cause and possible remedies. He said they were probably due to poor circulation in the legs. He also said certain mineral deficiencies may have something to do with it although research had failed to support this theory.
For myself I have definitely noticed that I'll pretty reliably have a night of leg cramps after a busy day at work (my job involves me being on my feet most of the day). This makes sense because muscle fatigue is characterised by a build up of lactic acid which is one of the causes of cramps in athletes.
My Ob suggested I see a reflexologist as he'd read of a study reporting its usefullness in preventing (non-pregnant) cramps. He was thinking of setting up a study looking at whether this also helped for pregnancy leg cramps. I can't report whether it has worked for me as since seeing him (when I was getting them several times a night, every night) I have actually only had a few. It may be because, as I'm getting further along in the pregnancy, I sitting down at EVERY opportunity- and work hasn't been so hectic.
Good luck finding something that helps.