I find that the best way to learn system design is to design systems -- and I have a pretty broad definition of "system". Most people see this area as a business-oriented thing; but games are systems, and we speak much of the "political system", "economic system", "educational system" and so on. There is more than one design for these things, and I would suggest that to complete your understanding of system design, compare the systems in your daily life, describe their patterns, and see what makes them tick.

Not everyone can build a better system, but knowing how these systems are built, how they're supposed to work, and how they do work, gives a much better idea of what's currently right and wrong with the world at large. It also allows for a better grasp on how to make things better.