Short blog post here. Two words that have consistently made a HUGE difference in the sound of my bands have been "steady air." For whatever reason, the imagery seems to be there for developing musicians, and it has helped to sort out so many tone/sound issues.
When making this point, I always go up to the board and draw the above image. I make sure to be extra dramatic when drawing the "not good air" portion. Seems to always get the point across.
The musings of a composer that also band directs!! ... or maybe it's the other way around..