Recalling my education days... there are different "skills of mind". Each one has to be trained differently.
If you want to increase focus on repetitive tasks, I would think you should do repetitive tasks and test yourself for accuracy. If you want increase short term memory, there are scientifically developed ways to do this. If you want to learn critical thinking, take a position that you wouldn't ordinarily take and then defend it rationally.
The human mind is adaptable; it does a very wide range of things. If you want to improve some task or another, just practice that task.