Archive, don’t delete. If you aren’t already, get in the habit of saving everything. Save all your emails, homework assignments, old quizzes/exams, etc. There have been countless incidents where I find myself working on a problem similar to one I solved in the past. Rather than attempting to either remember how I previously solved it or trying to re-solve it from scratch, I have been able to simply pull up my old solution. In the case of email, being able to reference old conversations can be helpful when trying to remember what decision was made and the rationale behind it. Storage is cheap, your time is valuable. For what it is worth, most of your classes follow this philosophy as well. They usually keep old homework assignments and exams available on their websites. If you already took a class, you can even pull up your old homework submissions from the class’s subversion repository.