Desirable Difficulties

Desirable difficulties are actions that appear to work worse in the short-term but work better in the long run.

These include:

Spacing. Practising something ten times in a row vs. ten times spaced out (over hours or days). The latter feels harder but results in more permanent memory.

Variability.  Intuition argues for mastering one thing before moving on to the next, but research suggests otherwise. Variable practice tends to result in better retention and transfer than blocked practice.