I'm about to start having lessons on the Alexander Technique, because I've had problems with my jaw (and i'm quite tall so posture while standing could do with a service too). I've heard it's help so many musicians, but haven't spoken directly to anyone involved in it. Is there anyone here who has tried it?

Haven't heard of it but would be interested in knowing what it is. Can you elaborate?

From what I understand, the Alexander Technique isn't specifically for musicians, but involves teaching you to do normal things like sitting and walking without putting any stress on your body. It teaches you balance, co-ordination, and if you have a medical problem it can relieve some of the more painful symptoms. I read at one point about how we're born naturally with the AT, but lose it as we get older. This explains why babies can balance their head on their neck perfectly when they're not in proportion. It apparently helps everyone from musicians to builders. I'm going personally because I've had a problem with my jaw threatening my bassoon playing, and my future bassoon teacher strongly recommended me getting lessons. I'm also quite tall, and my family has been moaning about standing up straight, hehe.
I haven't started yet but it sounds very promising. If you're interested you should look into it and maybe get some lessons. Might save you from repetitive strain injury or something...