Spacing, and mixing problems up

The same total hours produce substantially more retention when spread across days than when massed into one session. Revisit a topic a day later, then several days later, then a couple of weeks later. Each return is quicker than the last.

Mix problem types within a session too. Doing twenty of the same kind teaches you to execute a method; mixing kinds teaches you to recognise which method applies, which is what an exam and a job both actually test.