Over on Quora, this question gave me the chance to talk about my favourite piece of design advice:
“The best advice I ever got on design process is: Design for the most common task first and make that task simple. Then refactor your design for less common tasks, without making the most common tasks any harder.
A related concept is to design for mobile devices first, then refactor for desktop. Thinking about designing for “one eyeball, one thumb” will force you to decide what’s important.”