There are some good papers on the web about loglines. I'm going to keep mine short. Right now there is someone who is asking me to review their logline and again and again they keep on wanting to explain random details about their world when, in reality, almost all movies can be easily summarized with the following sentence.
A _______ must ______ before _____.
Call this technique: "A Must Before."
A young boy must stop the evil empire before it takes over the universe.
A Hobbit must destroy a ring before darkness rules for eternity.
And so on.
Start with that... then you customize. Ask yourself or have someone else ask you questions... the most interesting qustions can be added in as you expand your logline into a synopsis. But you should begin small and work up and always refer back to your controlling spine.
I'll make a quick one up now.
A boy must find his lost dog before it is eaten by wolves.
Of course there are more characters and more stuff happens and such - but this is where you start. You can even start writing movies from this point. The questions might be - who is the boy? How did he lose his Dog? Why are there wolves? Does he find the dog? That's all synopsis or story development stuff.
In science fiction people tend to have an even harder time because they want to explain their world. The world doesn't matter. We don't watch movies about worlds, we watch them about characters.
Science fiction sample:
A female rocket pilot must break through the alien guard that surrounds Earth in order to deliver a special message to a sympathetic alien that will change the course of this planet's history.
I didn't explain anything about the rocket's technology. I didn't talk about how the Aliens arrived or specifics about their guard. It doesn't matter. All that matters is that there is a person who has a goal and a time limit.
Once you adhere to this, you can make up stories left and right. Expand the ones which catch your interest. I may expand that last sci-fi one actually. :)