I have two one-line pitches, of sorts, which I used as opening and closing lines in my 90-second pitch sessions at the Writer's Digest pitch slam.
The opening one-liner is:
Sometimes you're already committed to the wrong person when fate finally brings you the right one.
And the closing one-liner (which stands alone as a one-line pitch, too) is:
Daniel and Marienne's friendship helps them weather every hardship from his divorce to her unexpected widowhood, now they're both both secretly wondering if it can survive a first kiss.
In between those two one-liners I put a description of the MC's, their situations, and the basic plot line.
For both of these I had to think about my book in terms of what would be put on a poster to advertise it.
Passionate Plume 1st Place Winner 2012 - ALWAYS YOU
Published with Ellora's Cave, Turquoise Morning Press & Samhain Publishing