My own view is this, and it is based on a lifetime of having made all the typical mistakes.
Steady is the best way to go for your investable funds. That means, go with stocks for a decent segment of your investments but temper this with investments that will help preserve capital when things get rocky. Keep a ratio between the two that is age-appropriate. There is a rule of thumb floating about among advisors that your stock percentage should be 110 minus your age. This may or may not be a good ratio for you but some method that helps discipline you in these decisions will help you and this is not a bad one for many people. The other major factor is to avoid impulse buying or selling and to keep transaction costs at a very low level - and this usually means going with broad-based no-load index funds for much of the ride.
Doing the above will not make anyone rich. It will, however, ensure that you have the best chances of getting decent, normal returns on average over time while helping to preserve your capital as you go. If you want extraordinary returns, get them through your startup or by doing extraordinary things in your work. For your investments, the rule is different. You do not "underperform" by hitting averages with your investments. You simply meet the goal that should be the defining goal for most people in that area.