Your head should be level, with your chin parallel with the floor. Keep your shoulders back and your chest high but not too high as to cause strain.
Keep your hands should be relaxed and still. Your feet should be set apart, with one slightly in front of the other so that a bit of body weight is put forward.
Keep the knees loose; they should be a little bent and never locked. The abdomen, however, should be kept flat and firm. This will keep it in an expandable position.