My sitting position uses one concept: the 90 degree rule. Your feet should be flat on the floor. Your knees should be at a 90 degree angle to your feet. Your legs should be bent at a 90 degree angle at yout knees. Your trunk should be at a 90 degrre angle to your legs,bent at your hips. Your back should be straight, and your shoulders parallel to the ground. Then i like to use items such as an A-frame stand or a cushion to prop the guitar up on the left leg. This a position that will give your body more strength.
Another item that I use quite often is a guitar strap. Of course, I don't use the strap with an expensive classical guitar, but on cheaper guitars i have found it very useful. In pit Orchestra situations, a strap is often practical because of space considerations. In the classroom, I am constantly walking around the room with my guitar to help students. I often use a strap when I am playing gigs. In more formal classical guitars performances, I will use the a-frame guitar stand position on my left leg.