PNF is fantastic, particularly if you have a SENSITIVELY AWARE stretching partner. As a Shiatsu student a huge part of my repertoire involves soft tissue work and communicating with the receivers body via Proprioception. If you're looking at working specific muscle groups it's wise to keep the intensity of the contraction at around 20% of the available force within the muscle. Going over this can fire up synergistic muscles to help support the tension being placed. Synergistic muscles are secondary muscles that help support the effort of the muscles being employed. When they become employed, particularly when dealing with rehabilitating a specific muscle/muscle group, the synergistic muscles may compensate for lack of performance in the agonist/antagonist muscles you are targeting. This causes a more generic response, where you may want a very specific outcome. The important aspect is the relaxation and mobility/stretch actions done after the initial contraction that does the magic. With PNF we're literally tricking the brain to release the muscle.
Soft tissue awareness is of paramount importance to understand for the MA with long term training in mind. In particular the myofascial system the holds us to together. Here are two great articles:
This one was posted by Ashe in his blog:http://www.t-nation.com/readArticle.do?id=1813999
And this a great one on myofascial meridians and the routes they take over the body.
Zach_Zinn if you're studying massage this is a great book that I'm reading at the moment:http://www.amazon.co.uk/exec/obidos/ASIN/0443071586