I have had three terms worth of Alexander Technique lessons with Lynne Cartlidge (I kid you not) in Cardiff. I also went on a residential weekend, which was like being on holiday with a bunch of extremely pleasant lunatics. It has certainly improved my use of my body, not just in playing but in many areas. My son, who is a professional music teacher, found it helped him when he had to learn to play the banjo and the mandolin in a short space of time. The downside is that it is expensive. I'm taking a break from it because of the expense, but will probably return to it in the future. In the lesson it's easy to hear and feel the difference, but replicating that at home is a different matter. It's all about being relaxed and having some understanding of how our bodies are actually put together - for instance, that muscles contract when they are in use.
In summary, it's like going on a fantastic adventure without leaving the room.