I will tell you how I come up with some of my pieces! It may come as a shock to you, but maybe not. I wouldn't be surprised either way, actually. :)

I love to turn my Itunes on shuffle and draw a picture reflecting the song that comes up. It's a nice creativity boosting exercise that gets your juices flowing to come up with bigger, and better, projects. 

I also like to illustrate the stories I write. I once read an article saying that it's rare to find an artist who can write well. This was quite offensive to me, since I writing and art are like oxygen for me. But also because of other great artists like Dr. Suess. Seriously, had they never even considered professionals in the field? Most of them write as well as draw. *sigh* sorry about the rant. It was unintentional. ^^;

Another great exercise is to draw right before, and right after you wake up. That way you get to tap into your subconscious right before you sleep, and when you wake up you can do a few quick sketches of your dreams. Just be sure to keep a sketchbook nearby because, if you're like me, your memory won't last long enough to walk across the room to find the darn thing. This is sort of like a dream dictionary, and it is beneficial in more ways than improving your art! You can learn a great deal about yourself and your life when you pay attention to your dreams. I would know (I wrote a paper about it). 

This isn't an exercise to generate ideas, but it is one that will improve your abilities. I do gesture drawings for at least 1 hour everyday. Usually after breakfast, I sit down and get on posemaniacs.com then just grab my charcoal, some newsprint paper, and go for it. This helps your ability to sight proportions, get used to the medium (believe me, that in and of itself is a chore), and if you so choose you can build up the values. Which will help you very much in the long run. Learning to see the negative spaces in the figure is an awesome skill to have. It makes drawing SO much easier. 
Practice, practice, practice. That's really what it takes.

Draw everyday. It doesn't matter what it is, or where you are. Bring a small sketchbook with you everywhere you go, and if you see something interesting or intriguing do a quick study of it. It could be the way that crack looks on the sidewalk, the mother holding her son's hand while they stroll through the park, the way the light shines through the leaves on a sunny day, or even the rain droplets that land on you window. There's beauty in everything if you just take the time to see it.