When it comes to guitar playing, there are a number of posture rules that should be considered in order to maximize efficiency. It is only natural that players from different traditions employ their instruments in different ways, though it should be noted that the classical posture is highly effective in terms of reducing fatigue, avoiding strains, and playing comfortably.
The classical guitar posture can be summarized in a series of rules that may not seem significant by themselves, but when put all together can really improve your relationship with the instrument, helping to make the most out of your practice sessions, and ultimately benefiting your overall sound.
I. You should sit near the edge of the chair, so that if you look directly down you can see the floor and not the sit of the chair. Avoid slouching. As for the chair itself, it should have no arms and, if possible, have its seat at such height that when you are sitting, you upper right leg is almost parallel with to the floor. Some guitarists measure this by standing up straight besides the chair and checking that their knees are at the same height as the seat.
II. Your legs should be at about shoulder width distance, with your right knee pointing at 45 degrees to the right, and your left leg point just a couple of degrees to the left. Of course, the final position of the legs is different for each guitarist. Remember that, in any case, the important point is that your body should be supporting the guitar in order to leave your hands free to play.
III. Use a footstool to raise your left leg. There are a number of traditions when it comes to the footstool: you can either use the tip of the footstool or place your whole foot into it, which results in a firmer position but reduces your ability to move it. Some guitarists use a steep slope in their footstool, while others prefer a minimum inclination. The best way to deal with these options is to experiment with each one of them. There is an alternative to the footstool: some players consider that artificially raising your leg ends up causing back pains because it twists the spine, so they employ guitar supports to raise the instrument instead.
IV. Once your legs are in position, the guitar's lower part will rest against your left leg, with the headstock approximately at eye level. The right forearm should make contact with the guitar's body just below the elbow. The weight of this arm will help to hold the instrument in place.
V. It is important that both arms can fall off naturally to the sides before raising them to play. The left hand should be able to move through most of the neck without having to drop the left shoulder. The correct posture for the right hand is a immense subject on its own, but a good starting point is to have the fingers flexed at every joint.
Remember that, in any case, your playing position must be comfortable in the long run. It is important to understand the difference between an immediately comfortable position (you can be more comfortable slouching, for example) and a position that, after extended periods of practice, will not hurt your back or give you severe pains. It is worthy trying to adopt the classical position then, even if at first it feels unnatural or just strange.
It is also advisable to remember that adhering to a certain tradition of sitting and positioning your hands and body doesn't necessarily imply that the guitarist limits his/herself to classical music. The classical posture is practical for a multitude of styles, and some players even adapt it for electric guitar.