An english professor of mine would often have us do these "free write" assignments. For these assignments we were asked to write "garbage". It didn't have to make sense or be cohesive we just had to write "junk" as she would call it. We would do this for around five to ten minutes and try to write as much as we could. I was amazed at how fast we were able to come up with ideas, that, in my opinion, were quite good.
I think that the same activity could applied for us artists. If you can't think of an idea just draw "junk". It doesn't have to be pretty or make any sense but keep your pencil to the paper and do not... I repeat do not erase! Keep the pencil, marker or whatever you are using moving. The sketches can be of whatever... just keep drawing. You will be amazed at what you come up with! Lets call this drawing exercise "Brain Fart Sketching".