An important point (underlining what the others have said, and paraphrasing Donald Knaub): Don't just blow air *at* the mouthpiece, but rather *through* the horn. You are not playing for yourself, or even the person waving their arms in front of the band; you are playing for the guy in the cheap seats: open up, move air and project. Go for good sound over loud. Breathe as much as you have to - cheat like hell, a lot of the greats did and do. You're blowing through 18 feet of big brass pipe; resistance is futile.
[Just sticking someone on the sousaphone (or marching tuba, whatever) with little/no instruction happens. We had one or two "horn holders" in my high school marching band who were recruited just to hold and march the souzys for formations, etc. and were not expected to play (had to have 8 or ten or whatever was called for visually).
]Eric
