He came up with his 16 Rules of Financial Safety, which incorporate a lot of common sense. A couple of these are not directly relevant to a DIY Income Investor, so I'll try to boil these down to just 10 rules [my few additions are in square brackets].
These rules are based on the text quoted on the Crawling Road blog, which links to a 'permanent portfolio forum' - worth a look, particularly for US visitors!
I'm sure there are a few more 'rules' that you could think of - but I think those above make a good start!
I am not a financial advisor and the information provided does not constitute financial advice. You should always do your own research on top of what you learn here to ensure that it's right for your specific circumstances.