First, toss everything you know about pacing. Play-by-post / play-by mail games tend to consist of long periods of low posting rates, punctuated by short periods of frenetic activity. Frame your scenes with as much detail as would be readily apparent to the PCs, give the players the information (and freedom to improvise somewhat) they need to portray their characters as the competent people they are. To effectively maintain dramatic tension, develop your scenes as if they were chapters in a book. Don't let a failed die roll kill the scene. Failure needs to be as interesting as success. In order to keep your players interested, the novel that is your adventure needs to be a page turner. Boredom tends to kill games quickly, as does too heavy a posting commitment unless all your players type a hundred words per minute and are already gifted writers.
“In our every deliberation, we must consider the impact of our decisions on the next seven generations.”
-- Great Law of the Iroquois Confederacy