8 of 101+

I have a page on this site entitled 101+ ways to sound better playing harmonica which was inspired by a Guitar Player Magazine article similarly titled. I'm working my way through each of the ways and elaborating on the point.

8. Don't tighten up - keep your upper body, neck and face as relaxed as possible. It will open your tone.
Good posture is important - for some of us, it's as important as good hand technique or even good embouchure. Stand to play when you can, only sit when you must. It's easier to breathe correctly (from your diaphragm) while standing. If you must sit, find a chair that allows you to keep your upper body upright but not rigid. Most folding chairs fail at this. Straight-backed wooden chairs are usually best.

Breathe from your diaphragm, from your guts. Breathe in a relaxed manner, but deeply, and don't try to hold in your gut. As you breathe and play, you may find that you can 'sip' some air to keep your lungs partly filled, or you can 'vent off' excess, by opening your mouth between notes and 'breathing (just the tiniest bit) around your harmonica. Takes practice, you'll get it.