Letting yourself get (very) wet on a rainy day.
I grew up with a dad who gave me two pieces of budgetary advice. 1) "You've got Champagne taste on a Budweiser budget" and 2) "I sure hope you're putting away some rainy-day money."

Maybe you've gotten similar, though differently worded, advice from a fiscal expert—say, Suze Orman—who pointed out the importance of creating an emergency fund. Maybe you also told yourself you couldn't afford to save for such a fund (right now); or you really, really needed to spend that money (on something else); or, if you were really careful and hoped (really hard), nothing would happen. Until finally...something did. The rain day arrived and you got soaking wet. You needed a new car to get to your new job and you couldn't get one and the offer went to somebody else. Or: You wanted to break up with your boyfriend and move out, but you needed money (fast) for a deposit on a studio apartment that you didn't have. At this point, the money meltdown was no longer about the money you lacked. It was bigger and uglier. You had to stay in a job you didn't like. You had to stay too long with somebody you needed to leave or you had to move onto a friend's lumpy, moldy, stinky basement couch. You felt trapped and frustrated and sad, but also—once you finished yelling at yourself—a little wiser. Because spending a few weeks on a lumpy, moldy, stinky couch (trust me) will keep you from sleeping more than an hour each night—providing you with plenty of extra time to figure out how you will spend and save your money very, very, very differently in the future.

Next: 6 money mistakes everyone makes (at least once)