I've found John's tip about raising the center of the tongue to be helpful on the D flageolette. Yesterday I actually managed to play it up the scale from D to high B. This is not something I would care to dwell on in thoughtful solos, but as a way of playing with the group and having a pleasant time, it gets the note out.

Wikihow suggests putting a piece of tape halfway or one-third of the way across the hole at bottom end. That seems to help. Letting the top index finger leak a bit also seems to help.

Also, I recalled something from the C recorder - to get a high B, play the fingering for a G#!