I'd say the very first concept to grasp in software design is separation of concerns.  Divide and conquer.  Eat the elephant one bite at a time.  Learning how to decompose a bigger problem into a series of approachable goals.  I'd rather work serially on a screen by completing one simple task before moving onto the next instead of working with all aspects of a screen in parallel.  Division of responsibility for easier programming is a major consideration by itself, but there's another piece of motivation almost as important.  Because user interface code can be very complex to debug and is prone to change based on user experience, I really, really want to extend granular unit testing with automated tests as far into the presentation layer as I possibly can.