My friend Dan Gage showed me this, and I've seen something like it described on Pat Missin's page. Pat describes some very esoteric techniques, but this one is useful to Blues players (- you players will want to try this if you don't already use it).

Play a 3-7 octave (blow).
Cover the 8-10 holes (I use my right cheek).
Expose the 1 hole.
Make sure your 'cup' is air tight.
You should hear the 1 DRAW sound - a nice 5th interval to your octave.

I wasn't able to get it at the time Dan showed me, but today I got it by accident as I was playing a Golden Melody - the covers are easier to get a tight seal around than my usual Special 20.