Plotting: How do you pace your story?

I sold another book! If you've read Restore My Heart, you'll know that one of the hero's siblings is at loose ends at the end of the book. In the ensuing two years, a lot happens and ... well, you'll see. The new book's working title is Reclaim My Life and is a Witness Security Program story. The manuscript is due Dec. 1, and the tentative release date is 2009.

This is one that I haven't actually written yet, so I'll be very busy the next four months. I try to write in a 3-act method, like a stage play. I hope to have act one finished by the end of August. I absolutely must be at the mid-point at the end of September, which is that event in Act II when everything shifts.

Any of you writers use this method? It seems to help my pacing, although it's really just another version of the story arc. If you aren't too busy meeting your own deadline, comment on what tricks you use to write toward your deadline, whether it's a contractual one or self-imposed.

Thanks!
Cheryl