As with most instruments, there are a multitude of dissenting opinions and different schools of thought on how to go about using your body to make music on the guitar. Lots of great players have very questionably efficient ways of doing so, but they make it work for them. The fact is that there are some very gifted people in the world, who will almost find a way to excel even if their own technique presents an obstacle. Excelling in spite of these is one of the defining features of a virtuoso, so imitation or 'modeling' isn't always a reliable tool.
The thing to remember is that when we're speaking about technique, it's often from a very idealistic point of view. In real world practice, the road has bumps, and the mind wanders. As a student looking to progress, you need to strive constantly for these ideals, but always accept and be aware of where you are.
The goals of technique? We want to increase the odds of playing with less effort, resisting injury or undue stress, and presenting the best opportunity to increase our facility and general comfort level.
To accomplish this we have to realize and accept that playing guitar is a whole body activity. We hold the instrument in position a certain way, and maintain a certain posture. This can affect the ease with which either or both of our hands have access, the level and location of extraneous tension in our bodies, our breathing, and indeed the security, comfort, and pleasure we feel as we try and make music.
Classical guitarists sit on an armless chair, with either their left leg elevated by a footstool, or the guitar itself elevated by a one of the *alternative guitar support devices. This elevation is intended to create the optimum position for left hand access to the neck, while playing complex polyphonic music.
The guitar is supported by the player's left thigh (or support device), chest, right inner thigh, and right arm. This system must be solid enough so that the left hand can move freely without being much of a factor in the support. In advanced players, the support of the right arm must also become rather fluid so that changes in tone and artificial harmonics can be accessed while the guitar remains stable.
Tips:
1. Take some time before you start practicing to examine your general sitting position in a mirror. Above all, comfort and ease is the most important goal. Don't always judge and scramble to change, sometimes just observe and understand.
2. In general, your spine should be neither bent nor twisted unduly.
3. Ideally your left thigh should be parallel to the floor (or angled slightly upwards) . This is a function of the seat height combined with your leg length. It's possible to make this happen when using a seat that's too high, by just setting a footstool higher, but if you do that excessively, it will cause your pelvis to twist asymmetrically and over time put strain on your lower back. There are certainly times you just have to make do, but finding a compatible seat height for longer periods of practice is very important.
4. Shoulders should be relatively symmetrical in height and as relaxed as possible.
5. Breathing should be free and unrestricted. if your breathing becomes shallow or held, this will ultimately affect your playing and expression.
6. Avoid chairs with a rear inclined sitting surface. Ideally it will be flat or even pitch towards the front of the chair. These tend to ease your spine into it's natural S shape and relieve lower back strain.
7. Avoid crooking your neck and intensely gazing at your hands. If you look do so from a distance.
8. Avoid positioning your guitar neck excessively high. Usually I recommend the top of the headstock at eye level as a maximum. Going much higher makes it less possible for you to use the weight of your arm to assist in left hand function. (more on this later)
9. Guitar not feeling securely held? Examine how well you are using all the support points: Left thigh (or support device), chest, right inner thigh, and right arm.
To close, I should stress that sitting position is highly individual. These are good fundamental guidelines for most people, but we are all built differently and thus relate to the instrument a little differently.
Never forget though, the way you choose to sit can make a big difference in your development as a player. As a young student, I used to spend 5-10 minutes before each practice session working on it myself.
Questions or comments?
Check out John William's footstool based sitting position here back at Guitar 87 in Toronto. This was a great concert and the first time I ever saw him play.
http://www.youtube.com/watch?feature=player_embedded&v=f5-bQbqfwHc
Until next time kittens! Practice hard!
*Ergoplay, NeckUp, Janssen Guitar Support, Gitano, Dynarette guitar cushion among others. I personally recommend the Ergoplay Troster design for its adjustability and security.