A big rule in books I've read from John Bogle to Peter Lynch to Warren Buffet always comes down to one single phrase that has never left my mind (and has done me well): invest in what you know. It's hard to invest in what you know if you are using auto-pilot solutions like index and mutual funds unless you scrutinize the industries those funds participate in as well as costs and other investing tidbits that all investors should really take the time to know. After all, it's your money -- where is it going? Invest yourself in information and then let your money do what it needs to, right?

This article over at Forbes tells of Dean White and his journey of 80+ years, as he's gone from teenage hard worker to billionaire real-estate magnate who built his fortune in the billboard industry. With that division gone, his sole focus now is the apartment industry, something Dean knows very, very well.

Which comes back to why billionaires get to where they are. In most cases, it takes decades (except the tech boom recently) to find out which industries you know, how to make money in them and how to stay personally invested in the areas where your money is working hard for you. After all, throwing wads of cash into a gaping hole -- and not knowing where it leads -- is the mistake many investors make. There could be a money-copying machine at the other end of that hole or even a paper monster ready to eat all those bills. Which would you rather have?