I don't have a magic formula but I do have some suggestions that have helped me. These are not guidelines I've come up with on my own, but a compendium of articles, classes and advice I've picked up from many different sources.
What kind of things need to be in that sentence? Start with the inciting incident which is likely your opening conflict in your story, add the obstacle, the main character and the quest. Some people call those the 'who, what, where and why should I care?'
Be prepared to write and rewrite your sentence. Sometimes I start with two or three sentences to get all the things listed above included. Then I pare a word here, change a verb there, cut out anything I can and try to make those multiple sentences into one clear one. Don't be vague or coy. Make each word count and try to showcase your voice.
I like to come up with three or four versions of my one-liners and then combine them or settle on one I really like. If you're going to pitch to an agent or editor at a conference, work out your logline before you go. They will ask you what your book is about and you should be able to tell them in a concise powerful sentence. This also helps if you bump into an agent or editor at the bar or in the elevator. Be ready.
What advice would you give to someone trying to create their first logline? Where do authors use one-liners?
