I think of a man and then remove reason and accountability.  Sorry, someone had to say it.

Writing about men is hard for men, too.  I'm sure female writers vacillate between flighty-bubbly-happy-go-shopping-valley-girl types and hard-nosed-take-charge-Gloria-Steinem types.  Men have the same stereotypes and we fight against them.  Suppose that I write a cop drama.  OK, which one is the Nick Nolte tough guy and which one has the Eddie Murphy street smarts?  See what I mean: the stereotype is already prescribed.  The problem is not writing about the opposite gender; the problem is avoiding cliche.  The moment that any of the above characters reach the page, the game is lost no matter which gender writes them.

Since I write plays, I must have a clear idea of each character's objective.  Then, I have to decide how they'll go about meeting that goal and the degree of its success.  The goal is what puts them on the stage, their means is what gives them life.  I find that gender is seldom an issue by approaching it this way.  My only gender struggles occur when I say to myself, "a man/woman would never do that."  The nice thing is this: when I say that phrase, it would have to be an awfully interesting character that could pull off that action, so I find a way to make it work.  

Small example: I'm working on a play set in ancient Greece but I wanted the female protagonist to be strong since she will later cuckold her husband--not quiet a tomboy, but able to be decisive and demanding.  This naturally led me to place the story in Sparta where the women trained and lived like the men.  An added advantage is that Spartans were famed for their monogamy and the absence of homosexuality, which means that her adultery later on becomes a more dramatic affair than if I placed it in Thebes or Athens.  After making the setting change, her gender became a secondary matter and I think she really blossomed as an interesting person.  That's really the goal when we write about anyone, though.