FWIW - i found that playing didgeridoo was a fantastic way to learn how to circular breathe. The didge is really open, so when you have mastered that, and then switch back to trumpet, circular breathing is a breeze. And because you get used to moving so much air on the didge, on trumpet you can circular breathe in almost any register - before learning didge, i could only circular breathe when playing in the stave.
Its also a great way to warm down after a heavy gig!
this site seems to have a good description of how to play didge, for anyone that's interested:
http://aboriginalart.com.au/didgerid...ackground.html
cheers,
Adrian