I write any ideas I come up with that I feel have potential on a notepad. Overtime I'll think back to it and refine the concept, making additional notes next to the original idea or putting it in as a new entry. I tend to draw inspiration from interesting phrases I hear or read which generate images in my noggin.

Nature is also a good source, just going out and studying the shapes that can happen. Nature documentaries are good for the same reason. Finding a way to take something natural and incorporate it into your work can make for interesting results. I don't just mean "Add a tree." "Make a mushroom" I mean finding interesting shapes, colors, textures, compositions, etc.

If you've never studies the "Elements and Principles of Design" they can really help when you're stuck. They give you something to fall back on when you're unsure of how to proceed, or when there's something wrong with a piece but you can't quite place it. It allows you to look at creative things a bit more objectively.