Depends on how you learn, and what the material is. Stuff that worked for me, in no particular order:

• Write a cheat sheet for exams, even if you can’t actually use it in the exam
• Start homework the day the lecture that covers the material is given in class.
• Try to explain the subject out loud to someone else (real or imaginary). Anywhere you draw a blank when talking is something you need to refresh on. Repeat this until you get it right.
• For memorization - heavy topics, build an Anki flashcard deck

All of these techniques are variations on the fact that people learn by repeated exposure. the closer together the initial repetitions are, the higher the retention.