1. If it works — don’t touch it!
2. Read rule number 1 again.
3. If something works on the first try — there’s definitely a bug somewhere.
4. Don’t fix something on a plane while it’s on the runway. Apply this rule to programming.
5. Perform regular (and proper!) data backups, multiple times a day. No backup system is expensive — every one will pay for itself the moment you need it.
6. And read rule number 1 one more time, and don’t let anyone convince you otherwise.

This is all somewhat tongue-in-cheek, but very real in practice.