Oh, lots of ways!
1.) By reading a metric ton of other stories - comics, novels, watching movies, playing videogames, etc., etc. - and letting my subconscious mind work away at that stuff until it comes up with something.
2.) Finding some theme or idea I want to tell a story about (personal responsibility, how to handle unexpected turns in life, growing up, the consequences of lying, etc., etc.) and then inventing characters and plotlines that work with that theme.
3.) I collect reference pictures. Not always for specific projects (though I do that too), but just in general. See a photo of a neat-looking house, or shots from a fashion show with really cool clothes? Nab them and file them away in the reference folder. The well-organised, cross-referenced reference folder, with subfolders within subfolders within subfolders. I, uh, I take my reference-photos seriously. Because at any moment, any of those pictures may - together or apart - spark a story, and then I need to know where the damned things are.
4.) When all else fails, scribble in my sketchbook and see what I come up with. I am pathologically incapable of drawing something that does not eventually end up having some sort of backstory to it. I am in awe of anyone who can just draw "random characters", because I just can't. I feel like Oprah sometimes - "You get a story! And you get a story! And you!"