I'm no Alec Baldwin in Glengarry Glen Ross, but I have my own ABC:
Always Backup Computer.
My school-provided Mac seems to think it's fun to crash, then delete all user profiles on restart. This morning the tech guy "blasted" it, figuring a little scorched earth precedes a forest of flowering data.
If I hadn't saved what I'd done, I'd have lost everything. Again.
Back up, son. Back it up.