I find it helps if I hold the tip of my tongue between my front teeth (or bite my lower lip slightly), hold my breath, and gaze at a point in the middle distance while I'm doing this. A slack-jawed blank stare is the proper expression to shoot for here. Half-vocalized mumblings of the self-questioning variety may or may not help but I do it anyway.

Barked knuckles are said to be optional but always seem to be included in my version.