Not that I'm finished or even that what I have is any good--and certainly not that I know how to do this well--but I had a bit of breakthrough today in my quest to write a Synopsis That Doth Not Suck.
1) Go through the behemoth, 25-page, dry-as-dust synopsis I already wrote--the one that details everything. Write out a list of all the turning points †.
2) Chop out all the turning points that relate to the subplot.
3) Smooth the list of turning points together to create a coherent synopsis, adding plot points and explanations only where needed.
Ta-da! I now have something that at least isn't hideous and boring.
† A turning point, as defined in Robert McKee's excellent book on (screenplay) writing Story, is a point of no return in a scene. This is when something gets said or done that is irreversible.
For example, a couple having an argument is reversible. They can kiss, make up, and go back to the way they were before the fight. However, if one of them blurts out that they're having an affair, that's a turning point. The couple might still find their way to a happy ending, but the relationship will never be the same again. That statement created an irreversible change.
Turning points usually accompany one or more characters getting a shock or surprise. This is why even a dry list of turning points makes a decently readable synopsis.
Robert McKee actually suggested this technique as a way to create a pitch.