InkTip.com has a new article about writing loglines that get attention. It's a fascinating read, and I recommend taking a look at the full article here. But if you're in a rush, here is what a few successful agents told Inktip they're looking for in a logline.
Logline Writing Basics
This is one of those aspects of being a screenwriter that I always struggle with - compellingly summing up my story in a sentence or two.