I use this elevator pitch formula. Can't find the original source.

For [customer-base], [startup] is a [category] that [main-feature]. Unlike [competitors], we [differentiators].

I strongly suggest putting such a pitch in your news.yc profile. Here's mine:

For people with many pictures and little time, ourdoings.com is a photo-sharing site that organizes them for you. Unlike conventional photo-sharing sites, we make it easy to create an illustrated story of what you've been doing, and to share your story in illustrated email.