Write a backstory for your character under a History or Background heading on your character sheet and have fun with it.
Include things in history like where your character was born, their parents, siblings and children's names, and highlights of their life.
Include details about your character that you want to remember. Update history with anything you want to keep track of.
Add to history whenever you want more detail or to include recent events.
Include some history if you like about companion characters you play with your main character.