Keep a brief private record of useful episodes while they are fresh. Note the date, what you worked on, your contribution, what you observed, and anything you still do not know. You do not need a polished portfolio entry every time.
Respect workplace rules and other people's privacy when keeping examples. A short description may be more appropriate than copying a document. Review your notes occasionally to spot recurring activities or improvements. That record can support more accurate reflection than trying to reconstruct your experience from memory months later.