I think writers ought to have a bad habit.
Not an inadvertent one, like the poor social habits discussed earlier, but a carefully-cultivated one. A signature vice, if you will. Coffee, booze, cigars; expensive leather shoes for your or the opposite sex as your taste dictates.
Why vices? Character, mostly, I suppose — I subscribe to the school of thought that people are more interested in character flaws than character merits (cruel voyeurs that we all are at heart), and I don’t have a lot of faith in an overly-virtuous author’s ability to really get down into the dirty muck of the human soul.
Put it this way — no one can really identify with a really good person, but we can all identify with a really bad one. We might not have made his/her choices but we’ve sure felt the temptation to. And I think it’s healthy for authors to have had that giving-into-temptation experience a time or two.
I’d wrap up with a candid “reveal” moment of my own personal flaw, but frankly I’m a collector at this point. Maybe you’d better tell me which vice you’ve been nurturing like a beloved child, and I’ll try and give you some helpful pointers. Helpful, right?
After all, we should balance our vices with at least a little virtue here and there.