The correct sitting position to play the classical guitar is the easiest task to master, and guess what… you already know it.
Start by sitting down without the guitar – sit on a chair without arms, towards the edge, with a good posture, both feet on the ground, shoulders level, and eyes looking forward… you’ve got it! At this point have someone hand you your guitar and without disturbing your posture, move your hands into position.
The real challenge with the seated position is to maintain it.
The reason the seated position is so difficult to maintain is that when we are required to do new, complex tasks with out left and right hands it is often the tendency to move and contort our bodies. It is like when we grip onto something very tightly, so tightly that we are hanging on for dear life. We will often squish up our faces, grit our teeth and close our eyes. None of these actions really help the act of holding on, but the tension from the hands and arms spreads throughout the whole body and the effort being exerted is expressed in our face making us contort into all sorts of positions. The same thing happens with the guitar.
It is well worth your time having a look a some footage of classical guitarists. Notice how some move their bodies, faces, lean to one side, dip their neck towards the guitar or any other movements. Then have a look at guitarists like John Williams, Paco de Lucia and David Russell. These three guitarists are excellent at relaxing their bodies through the most challenging works. If they can do it, so can you.
The best thing you can do to maintain a good seated position and instill good habits it to constantly monitor your posture. You can use a mirror, video recordings, or ask a friends/teacher to observe your posture as you play. Only once you are aware of what your body is doing can you improve it and control it. If you would like to know more about posture I recommend reading up on the Alexander Technique.