Having just developed a bit of a sore wrist on my fretting hand I thought I'd pass some information on to everybody on what looks a good website for all things to do with posture and playing guitar.

http://www.tuneup.com.au/node

I'm not sure if my sore wrist is solely due to my guitar playing or if it's something to do with the fact that I've just laid a patio and also helped my daughter move house last week....
I've never done a heavy job so it's probably just all too much heavy use in the last few weeks. Time to ease up a bit just to be on the safe side.

Hope this info helps others.

I think there are good general guidelines to follow but everyone is built different. I've seen good guitar players do things that would hurt me if I did it that way... like fretting the sixth string with my thumb on bar chords. I know many people who do this but I just don't bend that way.

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I play a cheap electric guitar.
C.E.G.

My philosophy is this:

Playing an instrument is difficult. Why make it more difficult than it needs to be? You're a machine, moving your body parts from one place to another in order to transfer energy to your instrument and make sound. If you can make the same sound with less energy, then why not do that?

Always take a look at your hands / arms / neck / back / face and determine whether they are in an inefficient position or wasting energy. You can figure this out by making a video of yourself and seeing what you do. Try to make your body into a machine where each part doesn't strain too much or have to travel too far. You can learn this concept from martial arts, too.

If you were to work in construction, hammering nails all day, you wouldn't be raising the hammer high in the air above your head with both fists. Many of the strokes would miss, and you'd be putting the nails in at the wrong angle, bending them, denting the wood. You wouldn't use too weak of a stroke, either, because then it would take you 10 minutes to tap tap tap one nail into the wood.

You want to find that most efficient usage of energy.

-Carl.