My best advice is to constantly think of things that you think you'd be able to translate well to film. It depends on what you like as far as stories go. I always try to pay attention to people's conversations and if I hear a strange story or something that would work well in one of my movies, I write it down, no matter how small(I carry a small pocket sized notebook.)
I also like to read a lot, and while this may sound bad, I take small parts from books all the time. Whether it's a small character quirk or a start for an idea on a scene. I obviously never take actual plot lines or fully drawn out characters, but it's a good starting point for where you may want to steer your movie.
I think writing or story telling comes from the individual person, and you may find something that works well for you, so it's hard to give specific advice, but rather what each of us actually does.
Another suggestion is talking to your friends about story ideas, whether you have friends that are writers or not, many people have some unique stories that could be part of a film, or be the main outline for a film.
You can also talk to college writing students, as they'll generally be happy to have one of their short stories turned into a short.