Use a mirror
Playing any instrument requires a great deal of physicality. Our bodies often look for the easiest positions to hold, generally those that use the least energy but are not often the best for playing. I find practising in front of a mirror helps me check my stance, my hand positions and other key indicators that can prevent fatigue and injury over long periods of time. Singing teachers, for example, often recommend singing in front of a mirror to check posture and relaxation, tongue position, unnecessary movement etc.