The following is printed out and taped to my desk at the office.  Whenever I make a mistake, it can usually be traced back to one of these rules being broken:

Mike’s Rules of IT

1. Assume nothing.
2. Verify everything.
3. Work smarter, not harder.
4. Use your resources.
5. Know when to escalate.
6. Details or die.
7. Reproduce the error.
8. Prove the fix.
9. Always check the physical connection.
10. Empathize with the user.