Have proper posture while playing. Sit or stand up straight, keep you feet flat on the floor, and hold the flute a little below parallel. Organ pipes and voices too, all sound best when the air comes through! Your arms should feel relaxed, but they may ache after extended playing. Crossing your legs will often affect your tone if you allow you posture to collapse as well. So keep your feet planted a little apart and flat on the ground. Make sure you keep your chest and head in an upright position to keep your lungs open.