Well, this is right up my alley!
I've played classical guitar for about 17 years and at least three of my teachers have had severe problems with back, shoulders and hands. This has forced them to rethink posture and ergonomics. As a product of this I would say I have pretty good posture. I am currently in dental school and most of the students display horrible ergonomics, which will ultimately lead to early retirement, pain and huge costs to society. So, I thought I'd offer you some friendly advice on posture and ergonomics. Please pay no attention to the somewhat uptight tone that may present itself in the text below. I'm doing this because I think it would help a lot of players. Here goes...
1. Don't use a foot stool unless you're Michael Angelo of Nitro and plan on spending an equal amount of time practicing right and left handed. Slouching over a guitar with one leg slightly elevated will make matters worse. Think of factory workers who spend all day doing a gazillion repetitions of a single movement. Playing guitar for any length of time is no different. Instead plant both feet firmly on the floor and elevate the guitar by way of guitar rest. Theese come in many different guises.There are cushions that you put on one leg and then rest the guitar upon the cushion, there are supports that you mount on the guitar (often with a piece of adhesive tape). You'll find these at www.guitarsalon.com
. Search for guitar support or guitar cushion.
2. Keep your back straight, shoulders relaxed and your elbows close to your body. Think of playing guitar as sitting at a nice dinner table: don't slouch, bend forward, tilt your neck, spread your elbows ect. ect. Relax and for God's sake, STOP PLAYING IF YOU EXPERIENCE PAIN!!!
3. Your hands are extensions of your arms, so try not to bend them in weird positions. When I see most bass player's and some classical and flamenco guitarist's right hand technique drawing from the Francisco Tarrega technique, it hurts by just looking. Watch Garry Willis of Tribal Tech and you'll see a good example of an ergonomic playing position.
4.Try to keep the neck at an upwards angle like classical guitarists do. Why? Try playing a nice big sweep arpeggio in the 15.th position and now try to play it in the first position. Left hand feels different? It shouldn't if you keep the neck at an upwards angle. Again, the hand is an extension of the arm!
5. Practice makes perfect posture. I used to practice in front of a mirror to really see if I was doing it the way I way supposed to. Play for a minute or two then look in the mirror and see if you're still sitting correctly. This will take time and possibly feel very strange at first. At one point I felt like I was a beginner playing an instrument I knew how to play, If that makes any sense.
Playing should be fun and it shouldn't be a health hazard. Remember, pain is the body telling you to stop. You'd be well adviced to do as it tells you!