I prefer PbP (message-based) as well.
In my campaign, I post a new "Chapter" once a week. During the week, the players respond to events in the Chapter, to NPCs, and to one another. I will add updates as time permits. The next week, I post a new Chapter. While this method of game play requires a degree of patience and the occasional spell-checker, I have found it one of the best options for those with hectic schedules.