There is, I think, an interesting disconnect between the way we train our minds and the way we train our bodies. The disconnect begins in our concept of learning. Most of us think that learning means the acquisition of new information or skills. One moment you do not know, the next moment you have learned. You have acquired something. So far so good.
If what you have learned involves physical skill, like riding a bike or ice skating, we do not assume that learning is the same as training. You may have learned to skate when you were five years old: if forty years have now passed without you skating, we asume you will be rusty at best. At worst, we would expect to have to learn all over again. If you skated every day for forty years, what would you be doing? Would you be training your muscles? Would you still be learning new things about skating?
Most people accept that physical skills require repetition. You do not have to skate everyday to learn to skate; you have to skate frequently to be able to skate at a certain level. We accept that our bodies need conditioning in order to stay strong and flexible and perform the right moves in the right sequence. We might not know that our brains are also part of this conditioning: by providing the right stimulation in the right patterns, we allow our brains to strengthen the connections that allow us to control our movements.
We are less likely to accept that this strengthening of neurological connections is also necessary in mental activities. To learn something once is to learn it only for that moment. In order to have more reliable access to what we learn, we require repetition: repetition that reinforces neurological connections so that the map of the learning within our brains stays clear and constant. Learning that is not reinforced in this way breaks down (the "wiring" weakens and the protein molecules deteriorate). To learn is to revisit what we have learned, often forming new connections and increasing the mental territory assigned to that learning.
It is not always obvious that our education system is well-designed for this kind of learning. In fact, the progress children make through different grades creates the illusion that they are learning new things every year. This is mostly an illusion. In fact, they are revisiting skills, concepts and patterns over and over again. The changes in the children mask the ways in which the learnings remain stable. That's why it is difficult to write curriculum for writing and reading: the underlying skills are repeated with finer distinctions. They are not replaced by new skills each year.
What do you already know that you want to focus on learning better? What have you done once that you want to be able to do more often or more reliably?