Good question. It's been posed since before Plato, so answers will either deteriorate into sound bits and jabs or this could become Fluther's biggest discussion!
Moderation doesn't mean you can't take a risk or you can't "throw caution to the wind". It means that you define an excess (extreme) and deficiency.
Example: I want to be good at playing piano. How should I go about it?
Excess: Doing nothing but piano and neglecting everything else in my life, working my butt off every day and having no time for anything else (a.k.a. workaholic).
Deficiency: Doing nothing and expecting it to work out. Being lazy, kinda hoping, and letting things distract.
Moderate: Set a specific time every day where for 3-4 hours I'll work incredibly hard and stay focused on nothing but piano during that time. And then during the rest of the day, spend time doing other things keeping in mind that piano may be my goal, but it's not the only thing in life that matters.
Now I can take that moderate approach and set myself up so that all my financial income surrounds that (and maybe I'll practice for 6-7 hours a day) and throw caution to the winds in that my energies are focused on this to make it my life. The typical musician doesn't make much money, so that's incredibly risky. But if I'm balanced with it (working very hard to make it work but also remembering the other parts of life) that's a moderate approach.
Being moderate is defining the three aspects of a decision and acting up on that. It's not easy to do, but yes, I would define it as the answer to most everything.