What's your favorite study hack or technique that has significantly boosted your productivity?

I believe that hacks are overrated. The only hacks are

Study what you are interested in. You cannot consistently study what you don’t find interesting. You won’t even feel like you are working if you take up a subject of interest.

Take regular breaks between your study sessions. This ensures you have time to relax, and your brain can register the information you learned.

Solve more problems. You are likely studying for an exam, and exams ask problems. Problems will prepare you for a better score, and they also help you understand that theory better.