I can't find it, but I recall an article in Science showing that Myelin production is increased when there is a continued demand for it (so Myelin production can be trained like a muscle to some degree). The higher-level interpretation was that since Myelin production is connected to brain elasticity and learning, that once you get over a initial bump of starting to pick up the habit of learning more Myelin would be produced and it would be easier to learn going forward.
Anecdotally I found that to be very true. It was always easier for me to pick up new things during the week when I kept my brain in a "learning state" on my off-days by reading books or having a stimulating discussion with friends. But who knows, that might just as well be a Placebo effect.
(Sorry for missing reference, and the unscientific jumping to conclusions)