These are details for a current project; usually a colleague and I draw them up on butter paper or on the back of old tender documents. Other times, I try to record my ideas in my sketch book.
I enjoy the discipline and the solitude of this task; in the office I put my headphones on to do this without interruption. Flying times are handy for this too; 3-4 details between Kuching and Bintulu while Kuching - KL gets you 6. Mostly these ideas are already sorted out in my mind, only needing to drawn up for discussion, pricing, etc.
The more troublesome ones have a way of working themselves out once they are set free on paper. I show the built items in a month's time.