Bending

I am enjoying learning how to play blues harmonica.  So far my instruction is from two library books.

Apparently there are three main elements to blues harmonica: trills, rifts, and bending notes down.  (A fourth element is making use of the Circle of Fifths to play five half-steps higher than the harmonica is tuned for.  But the musician need not think about this.)

Bending is a very organic endeavor involving mouth shape that is difficult to teach either with a book or in person, since no one else can see or feel the size and shape of your vocal cavity.  The tips are thus interestingly varied.

The Complete Idiot's Guide to Playing the Harmonica, 2nd edition on page 120 lists its "Ten Tips for Bending Draw Notes"

1. Think your notes lower
2. Try whistling a high note and then a low note.  Then sing a hgh note followed by a low note.  Do you feel what happens to your vocal cavity and airflow?  You want to get that same feeling to bend your draw notes.
3. Stay relaxed.  Place the harp deep into your mouth.  Maintain a controlled, focused airflow
4. For starters, play a 4-draw and without doing anything else tilt the harmonica downward slowly.  You'll hear the pitch flatten.  You do not want to do this when bending a draw note.
5. Focus on dropping your tongue flat against the bottom of your mouth, lowering your jaw, and opening your throat wide.  If you feel your nose pinch shut that's good.
6. Purse (pinch) your lips slightly.  Don't overdo this.
7. Mouth a "weeeh" vowel sound while drawing the note and then switch to an "aauh" vowel sound.
8. Just think about sticking your jaw out there like Popeye the Sailor Man.  Stick it out and be proud.
9. Practice bending with both the pucker technique (easier) and tongue-block technique.
10. Do not breathe the air any harder.  Creating a draw (or blow) bend does not require more air.  Blowing or drawing harder only makes your bends more difficult.

For comparison, Blues Harmonica by Tom Ball on page 25 lists his tips, some of which are identical and other in direct opposition.

1. Say "oooh - aaah - oooh - aaah" to yourself while drawing the note
2. Say "oooy - oooy - oooy"
3. Raise and/or lower your jaw a bit
4. Shift your tongue back in your mouth
5. Pinch the wind supply by tightening the tension in the throat
6. Tense your lips slightly
7. Suck harder!

UPDATE: For the sake of comparison, here are four websites with their lists of advice.