I like to let a tricky problem breathe for awhile and think about it in the background of more turbulent tasks or on my walks. If all that's left when I begin working on it is organizing the solution into something real, I find flow pretty available.

I started on changeable territory billing at hour 10 in the work day but the flow was so strong I went four hours on it.

I think the main thing for me is not needing to task switch between research/investigation/ideation and organizing the solution.