Bill Pace (http://twitter.com/scripteach), a NYC writer/teacher/filmmaker, twittered 2 articles that he uses when he gets stuck at the Midpoint. They're worth downloading. Thanks to him for generously sharing with us!
The 1st is a list of helpful midpoint definitions & examples.
The 2nd is titled ACT II - THE ELUSIVE HEART OF THE SCREENPLAY by Jengo Robinson, a London script consultant, which analyzes The Graduate.
I really liked the latter article b/c it shows WHAT the midpoint's purpose is (heighten drama) and more importantly, HOW to ramp up to it and what to do after it.
WHAT I'VE LEARNED: Many scripts flatten in Act 2. Why? Often it's b/c they are not driving toward a midpoint where something drastically changes & now the protagonist is in an unforeseen mad, mad world.
(In the article above, Robinson calls this the "flip the script" moment.)