Breathing properly is the easiest when standing, because your muscles are relaxed and your stomach is free. When sitting you must make sure the thighs don't go upwards, rather downwards (the steeper, the better), so the stomach isn't bent in and the diaphragm can be moved down easily. This means you should sit on the edge of the chair, no leaning back, and the chair must not be too low. Some professionals bring their own chair. But that is true for all wind instruments. Better concert halls usually offer you chairs of different heights. I have bought myself an orthopedic foam support that you can place on your chair which makes sitting more relaxed. If you do, too, then make sure it is of a dark color so it does not look irritating to audiences