The venerable ad agency, J. Walter Thompson once (or more) published a full-page advertisement entitled, "Write and you’ll get work."
It featured a dozen or so very thought provoking – and daunting – questions for any aspiring copywriter. A talented writer could take months to answer them.
I wish I could find a copy of that ad. (I’m not looking to land a copywriter job at J. Walter. It was simply inspiring.)
Perhaps you’ve seen the interview questions at Microsoft? These might be for real, or not. Either way, they are good for the head.
For exercise, puzzle over one a day.