Inspired by this podcast.
Basically, a good story is usually something with something familiar as a premise, and then a new twist.
Now, an example that doesn't work:
Familiar: A highschool student
Original: He fights demons.
An example that does work (used in the cast)
Familiar: There is a stargate-esque transporter that lets you travel anywhere
Original: It causes clones of people to be made, and someone is abusing it.
It's a simple writing exercise. Try it out.