How Are Sitcoms Written?

I'm listening to the Thursday tele-seminar by sitcom guru Ken Levine, who runs sitcomroom as well as a fascinating blog on writing sitcoms, and offers seminars that teach you what it's like to be in the sitcom writers room. Ken has cranked out episodes for many classic TV shows including "The Jeffersons," "Mash" and "Cheers." He's fielding questions from dozens of us who phoned in to listen and ask about the writing process.
Some of his suggestions for beginning writers include, always begin with an outline, rather than jumping in to writing the script right away--it's too easy to paint yourself into a corner. Makes sense to me--I'm a big believer in outlining.
Establish your story in an outline first, and figure out your act breaks (space for commercials) and, most importantly, how your story will end.
Story ideas need interesting characters--on "The Office," we see a delusional boss who is inept in performing his job and wrongly believes he's the best boss in the world.
Supporting characters must offer opportunities for conflict with the central character.
What's the one best thing people can do tomorrow to further their careers?: Continue to write. If you have one good script, don't rest on your laurels. "If you write three scripts. I'm almost certain that the third script is going to be better than your first," said Dan O'Day, Levine's writing partner.
Added Levine, "Remember, no one can stop you from writing."