My three rules of life:
1) Whatever it is, don't put it up your ass1. You'd be surprised how many people do not follow this simple rule. I guarantee if you do a search in Google news for the word rectum, you will see someone dealing with the consequences of not respecting this simple rule.
2) It is always easier to ask for forgiveness than permission. This rule guides most of my professional activities and is most helpful when dealing with jobworths2.
3) You should always have three rules.
1I got this one from S_ who is an endless fount of wisdom.
2Most useful British-English word ever.