One thing I discovered from lifting weights is that learning generalizes, typically referred to as "carry-over". In weightlifting, this means that squats and bent over rows will both improve your deadlift. In general learning, this means that for instance learning about nonlinear dynamic systems, nuclear physics or crystal chemistry will make you a better molecular biologist. I've also found essay writing will improve your programming (as well as the reverse), and logic/mathematical proofs carry over a bit to both.
This carry over effect is important in particular because when you study one thing for a while your learning rate starts to suffer from diminishing returns. As a result, even though the carryover from another field might not be that high, you can actually improve faster in your main field by studying something else. Thus you can optimize your learning by rotating areas of study fairly often, only coming back to a previously visited topic when it's "fresh" again.
