I recently bought Sinta's book on Voicings, as recommended by my teacher. However, I'm having trouble with the high-F bending exercise (where you glissando down in semitones while fingering front F). I'm having a hard time often times getting below even Eb without the pitch dropping out. Several people have told me to visualize whistling while playing the exercise, but as I can't whistle, this isn't of much help. Any suggestions on how to work this out?