Sometimes great story ideas just pop into my head. Other times, I have to struggle to come up with one. (This is often because that great idea that came to me turned out to be impossible to, you know, execute.) During that struggle, I'll mine a variety of sources, including but not limited to:
1. My own life and people I know.
2. Things in the world that interest me.
3. Other movies.
That third one is inevitable for most writers, I think. Few of us like the idea of "stealing" an idea from another movie, but the more you write scripts and see movies, the more you come to realize that anything you write is going to have similarities to existing material. This isn't because it's 2010 and we're so starved for ideas that we're turning Mafia Wars into a movie; it's been true for a very long time. (Blah blah ancient Greece, Aristotle's Poetics, etc.)
It's okay, though. If you go about it the right way, you can extract a great idea from an existing movie without anyone noticing. It requires stripping away a lot of "story veneer" (for lack of a better term) to get at the core ideas that make the story special.
Novice writers often do the opposite, and keep only the veneer of a movie they like without touching any of the underlying principles. (This is also the case 99% of the time a studio chooses to "reboot" or "reimagine" an older movie.)
Example: Let's say you've picked Rocky to mine for material. Since it's such an obscure film, let's summarize it first. A small-time boxer and loan shark enforcer gets a shot at a high-profile match with the heavyweight champ.
To me, the "story veneer" on this is everything to do with sports/boxing. If you try to use Rocky as a model for another sports underdog movie, you're treading on territory that's so well-worn, you can see the concrete under the floorboards. And you may be more likely to miss out or gloss over the real heart of the story.
So what is the real heart of Rocky?
To me? It's about having lived through so much disappointment that your expectations are through the floor. You're at peace (or as close to it as possible) with the prospect of never meaning anything to anyone, including yourself. You'll just get by for as long as you can, and then you're done.
And then out of nowhere comes the opportunity to prove that you really are somebody -- and not just somebody, but one of the very best at something that truly means something to you. If you succeed, you'll be on top of the world, but if you fail you'll never live it down; you won't even have the luxury of toiling in obscurity, since everyone will know that you blew it.
So, if you're going to pull this off, you have to exterminate all the negativity that's been infecting your brain all this time -- learn to trust yourself, to love yourself. Build your self-esteem up to a level where it can be truly validated... or truly annihilated.
And then you have to take on the single hardest task that's ever been put in front of you.
Does that sound like it needs to be a story about sports?
Of course not. It could be anything from a romantic comedy to a heist flick. It's a general set of circumstances than affords a certain type of character the opportunity to become something much greater. There are a ton of ways to turn this into a blueprint for a great movie, without inviting any obvious comparisons to Rocky.
Notice that this story archetype, while highly flexible, comes with its own creative limitations that are helpful in funneling our thought process. For example, the protagonist clearly isn't going to be the President of the United States, or a superstar homicide detective, or the captain of the Harvard debate team. (If you want to put one of those characters in the lead, chances the story will have to focus on something vital being taken away from them, rather than the chance to get even more than they already have.)