Thought Provoking Subjects:
1.) Greek Mythology
2.) Psychedelic Music, including but not limited to: Pink Floyd, Porcupine Tree, Aphex Twin and Lords of Acid.
3.) Other artists
4.) Romantic/Gothic literature
5.) Philosophy
6.) Dreams
7.) The Underworld (Plutonian domain)
8.) Discussing points of view, and perspectives
9.) Cheesy horror movies
10.) Meditation
Brain games:
1.)"What if? Then..."
You start out with a statement, then twist it with a what if question, then answer it with another statement. Repeat until you find yourself journeying far beyond your original train of thought. This is a good game to play for writers block.
2.) "Freeze Frame"
When watching television, youtube or reading a story, try to freeze a particular motion of a character and take it out of context to make it more interesting. An example of this is you see a person adjusting their television aerials, freeze their motion and they're now wrestling with two giant snakes, or turning the wheel of a car with no seat... This was originally an improvisation game for actors but it works just was well for artist block concerning the actions of the human form...
3.) "Perspective"
Try to find alternative uses for things around you, or to make something into something that you think it is not. Draw a hexagon, now draw a line from each vertex to the middle. I could be slices of pie, a wagon wheel or a three dimensional cube (draw the hexagon and see if you can see the cube.) This is good for macabre artists or psychedelic artists...
4.) "Combination"
Close your eyes, now imagine combining the last two things you saw into something completely new. This is a good one for photoshop artists...
5.) Just put your pen, mouse, camera or brain to work. You never know what might happen once you get the wheels a' turnin'.
6.) Just talk it out. Sometimes two heads are better then one. Multiple thought processes can take an idea in completely new directions. Almost all of the Pixar moves were created in a single brainstorming session over lunch... Good for just about any style of expression...