First of all, set up some limitations. When you have no limitations, it is hard to find the one thing that you want to do. So if you set up some limitations, you can then focus on what can be made interesting within those parameters. I heard this story third hand so I'll probably mess it up, but apparently Stephen Sondheim said that if you asked him to write a love song, he wouldn't know where to start, but if you asked him to write a love song about a woman in a red dress standing under a streetlight in the rain, then he could do something with that.
One exercise is to set yourself a goal of coming up with 100 ideas of whatever it is you do. For example, write out 100 things you can do with a pencil. They don't have to be good ideas, but you have to come up with 100. If you want to write a story and let's say you've set up the above mentioned limitations, pick an element (maybe there is a dragon in your story) and write down a list of 100 things that dragons do. Or if you want to paint a picture or make a drawing, make 100 quick sketches of how you might see a particular object (Like Hokusai's 100 views of Mt. Fuji.) At any rate, the point of this exercise is to loosen you up and get you working without worrying about it being bad. The point is to limber up your imagination.
Steal other people's ideas. Look at what other artists do and if they have a good idea, use it, but then modify it so it is your own. Sometimes it can be useful to steal from an entirely different genre or medium of art. If you are writing a story, you might look at artworks and start thinking, "what sort of story could I weave from this image?" Or if you are a photographer, listen to some music and ask yourself how you might translate that into a picture.
Or you can just drink a lot of alcohol.
Lars