The trick is to have the person learn things that are not too hard, or too easy. This is hard to judge and changes with what the person already knows.

Finally, the students tested on the word pairs they studied. The students mind wandered more while studying more difficult and easy word pairs, compared with medium difficulty.

Xu, J., & Metcalfe, J. (2016). Studying in the region of proximal learning reduces mind wandering Memory & Cognition, 44 (5), 681-695 DOI: 10.3758/s13421-016-0589-8