After employment, I think that most individuals gain income diversification through investing.  It is important to look at why we invest: because at some point we plan on using this money for something.  For most, it is saving for retirement, and the investing is done through vehicles, such as a 401(k) or IRA.  But investing is not just about stashing money away for a rainy day – that is what an emergency fund is for.  Investing is about having enough capital to generate income.
Thanks for writing this Mr. Samurai. I just got over the student loan hump but I feel pretty good about it at 27 having a graduate degree and being 100% debt free. Now that I’m on the other side it is good for my brain to absorb some of your knowledge regarding passive income investments. I love gleaning wisdom from older folks who have been there and done that. Mentors rock!
An obvious example is over exposure to bank stocks, which have been excellent investments for over a century.  Though a foundation of most portfolios, bank stocks do involve more risk at certain stages of the economic cycle than many realise.  Being less exposed to bank shares in the last few months could have preserved some capital.  So, a more diversified approach can help mitigate some of these risks.