This is the question to dwell on I think. I don't know if there are mental ergonomics that apply universally, or even ones that generalise well. For me, I recognise that I think well when I'm physically engaged in some other non-cognitive task, like walking, running, showering, etc.

Another key factor is eliminating the risk of interruptions. I know when I'm at my desk I'm likely to get interrupted, but times and spaces that are interruption free seem to encourage the deepest thought.