Think like a programmer. The logline was originally developed to provide studio story departments with a simple description of a script. Users had to be able to tell as much about the movie as possible with just a glance. There's a fairly formulaic way to achieve this - and it has nothing to do with your movie being formulaic. A well written logline should read almost like a computer program. It is the details you choose to put into that structure that will make your logline sing.
Clearly define your genre. This is something you absolutely have to get across to readers. If you're thinking in terms of your genre as you go through the other steps, your word choices will begin to suggest the genre you're aiming for.
Clearly define your protagonist. Pick one word or phrase that is most important in determining who he or she is. For Indiana Jones, you might choose "archaeologist."
Give your protagonist an adjective. Now that you've got a noun, put an adjective in front of it to characterize him. You could call Indiana Jones an adventurous archaeologist, or a globe-trotting archaeologist,
Clearly define the objective. Now you need a verb. Identify the single most important thing the protagonist is trying to do in your movie. Indiana Jones travels the globe, he fights Nazis, he tries to mend fences with his ex-girlfriend.
Clearly define the opposition. Who's the bad guy of your movie? Who or what is trying to stop the hero? In Indy's case, the opposition is the Nazis.
Clearly define what's at stake. Indy is looking for the lost ark, and the Nazis are trying to stop him. What happens if Indy loses? In this case, the Nazis will use the ark's magical powers to take over the world. The stakes can be as simple as survival, or they can be more subtle.
Put it all together and see how it reads. Our logline for Raiders of the Lost Ark now looks something like this. "A whip-cracking archaeologist struggles to locate the long lost Ark of the Covenant before the Nazis can find it and use its magical power to conquer the world." If your logline's working, you should have a single sentence that tells someone who's never heard of your story who it's about, what they do, what they're up against and what's at stake if they fail.