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you just gotta keep trying it. Working at it.

Get any pitch going with the buzz, don't worry about specific intervals. It's a lot easier to sing higher than the buzzed pitch, for most people at least. Do it on the mouthpiece alone, and when you get a good, sustained sing/buzz, put the mpc into the horn.

There aren't any magic words, unfortunately, and no real universal tricks. But, once you get it, it never really goes away.

Once you've established your ability to play and sing, THEN start working on specific intervals, and getting them in tune. (That gives YOU a hobby )

hey...I posted the multi phonics thread!
But, the way you do it, is either hum or sing while buzzing your lips, as you said you heard. However, the way I had learned how to do multi phonics was on the Didgeridoo (It's spelled right...look it up...there's a million different spellings), and I learned it more in Australia while trying the didge's from a few of the guys there, and they are masters at that type of thing...But, that's how I learned it...Just keep practicing!

I find it easiest to start by playing a note that lies really open and esay on the center of your horn, like a G or C or whatever, then just hum away and try to keep the buzz going. eventually you can get actual intervals.

if you manage to multiphonic the fifth really really well in tune, the third pops out and you start playing triads! (Dave uNland demonstrated this for me as a way to get proper intonation for rochut etudes, my jaw hasn't come back into place yet!)