In Orson Scott Card's Characters and Viewpoint and How to Write Science Fiction and Fantasy, he this thing he calls A Thousand Ideas in an Hour. It's a fun exercise and a great way to get past writer's block. The idea is this. Starting with whatever idea you have, ask these three questions: How? Why? What result?
For example, you've got a princess locked in a tower. How did she get there? Why is she locked up? What happens as a result? Every answer is a branch. Some branches will end quickly, others will lead you into the rest of your story. Toss in a little, "What could go wrong?" and toss out anything that feels too cliche, and you've got yourself a story.
I did this once with a class of highschoolers, and it was their favorite part of the class. It went something like this:
Let's start with something simple. Give me an occupation.Around here we had to end the class, but you get the idea. Leading the discussion, I tried to follow paths that sounded more original and had more conflict potential, but any of these answers could be turned into an interesting story with some more work.
Okay, let's go with the banker. What could go wrong at a bank?
It could get robbed.
Sure. I don't think we need to ask why yet, so how might this happen?
A man walks in with a gun and asks for money.
Some men take the bank hostage.
Someone blows up the safe.
Someone inside the bank robs it.
Okay, great. Let's go with someone inside the bank. Who could do that? Who's inside a bank?
How could one of these folks rob the bank?
The guard could let other robbers inside the bank.
The teller could grab some money off the counter when nobody's looking.
The guard could raise a false alarm and, while everyone's distracted, go into the vault.
...or take money off the counter.
...or take money from someone's pocket.
What about the security guard. Why would he do that?
He hates his job.
He's been planning to rob the bank for months/years, and got hired so he could do it.
He needs the money for his daughter's operation.
Try it out and see what you come up with. Better yet, tell me how you brainstorm to get past writer's block.