Tim Bales

How?
Getting them to play the lead pipe of the horn. With me. 
We play some follow the leader things, riffs, where I play first, then they come in, and play with me, with the strong sound and full volume.  
Playing the lead pipe functions the same as swinging a weighted baseball bat does. When you remove the weight after the practice swings, the bat feels lighter and faster. 
Same with the lead pipe exercises. To get the big sound they hear me playing, they open their embouchure slightly, and push more air through the horn. 
After five or ten minutes of this, we go to playing the horn itself. I have to warn them, "It is going to be loud, so don't get scared and play softly. 
And, it works.