Slightly off topic:
I've read somewhere recently that Aaron Sorkin doesn't bother much with writing specific dialogue for specific characters, but that he likes all of his characters to be sharp-witted and quick; and it occurred to me that when I was watching West Wing I though how pretty much every character in it had clever and bright dialogue... but how funny and good it nevertheless was.

I am one of the "write what you know" types in that there is inevitably a character that is my age and/or acts exactly like me. So I put myself into the scripts almost every time. That character is not usually the lead, but interacts heavily with the lead because my personality doesn't make a very interesting lead character since I tend to be so logical and level headed. Who wants that?

So like I've often been on here, my character is the voice of reason within the midst of the script's madness. The lead is anything that the story requires. They tend to be younger males (sort of like I think I would have been if I had been more outgoing), and though I use females when the story calls for them, I acknowledge I don't write women very well and they are more like me with a sex change.

As such, I lean towards using males because they're more believable.

Theres a great section in the Stephen King book 'On Writing' where he talks about incorporating first hand experience into a story. If we base our story around certain aspects of our lives  (workplace, social environment, etc) then it gives us an opening point from where we can get to know our characters. From our own experiences we know all the details of how our character might be thinking or feeling.