Re: Posture when playing sitting down
I think traditional "classical" posture (if you're a rightie - sitting, fairly upright, left leg on something, guitar sitting on your left thigh, fairly steep guitar angle) is considered the best practice. That said,
1) I tend to rest the guitar on my right leg, both feet flat on the floor - it's just more comfortable for me, though it's probably more of a casual, "rock" playing position, and
2) since almost all of my practice is standing up, that's actually the position that feels the most natural for me - sitting down doesn't feel as natural.