I tend to wobble the bottom lip up and down slightly, so it gives a change of tone rather than a change of pitch. It's probably allowing more/less air through, changing the pressure. It is easily variable both in speed and depth, but certainly not changing the pitch. I daresay that pitch is also variable, but it's going to be a more advanced technique, with a great reliance on ears. There's nothing worse than someone playing a note, and wandering off it with a wide pitch vibrato - unless they know what they're up to!
EDIT: and before anyone says it's not right, I checked it against a tuner, and the vibrato is there, with NO change of pitch. Maybe it's unconventional, but it works for me...
EDIT 2: As pointed out correctly by Carl, this is far more of a tremolo effect than vibrato, although it is still effective.