Your metronome idea is good. It's the whole rhythm concept in your head. I don't use a metronome but I do go through sort of a standard check list in my head as I'm drawing and setting up to get my shot off. It runs quickly through my head as I am doing it but to a specific rhythm that I almost keep a beat too.
It keeps me focused and where I don't stop and dwell on any particular thing.
"And I saw heaven opened, and behold a white horse; and he that sat upon him was called Faithful and True, and in righteousness he doth judge and make war." ~ Revelation 19:11