Harry Browne was a US personal finance commentator and advisor. He is particularly well-known for his 'permanent portfolio' concept (although this is not a model I agree with totally!). One of his famous observations is: “The best kept secret in the investing world: Almost nothing turns out as expected.”
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].