I manage my own investments (which I'm kind of glad of considering the market). Now you may not be up for managing your investments, but there is one you should manage – your career.
Think about what an investment is – you put money into something (stocks, land, a business), from which you expect to get benefits that are greater than the money and effort you put into it. The benefits may not be the same (an investment of time may make money, and investment of money may yield fame, etc.), but the principle is the same – put something in that yields more.
Your entire career is a series of investments. Indeed your life is.