When I took my first APL class the prof drilled into our heads to focus on data representation before writing any code. Over the years and across a dozen languages or so this has proven to perhaps have been the most valuable thing I learned in CS. I lost count of how many problems I've seen go from difficult to manageable and even simple to solve by devoting more effort towards finding the best way to represent the data or the problem than simply taking things as presented and writing code right away.