Here's a trick I've been using in some lessons. Buzz sustained notes with a ridiculously wide vibrato...one might say a Goofy Vibrato.
I think this helps in a few ways:
- It helps you develop your normal vibrato.
- It helps you find the center of the note.
- It strengthens the embouchure.
Why? The embouchure corners must work harder to maintain the pitch when everything is shaking around.
Make sure that, as you buzz with that wide vibrato, you don't lose the center of the pitch. Also, make sure the air continues to flow.
You can also try goofy vibrato on the trombone (stay relaxed, keep the air moving!)