Maximize Expected Returns
This one seems obvious. Given two approximately equivalent investment options, choose the one that offers higher expected returns.
If you have a lump of money that you expect to need in the short term, put it in a savings account yielding 1%, not a checking account yielding 0%.
This rule applies to debt, too! If you have the choice between investing money at a 7% interest rate or paying off debt at 2%, you're probably better off investing the money and making minimum payments on the debt (for a net 5% return).