The first and probably better-known are methods laid out by Austin Osman Spare, and I have a link to his essay on it with Frederick Carter, here -- Automatic Drawing. The pictures don't get any better upon printing, I'm afraid, but a quick search can find you loads of Spare's drawings online. I understand some of his books complete with art are being reprinted. His full texts, sans art, are readily available free online, but without the drawings it doesn't seem like there's much point.
Automatic Drawing also touches on Spare's methods of making sigils.
The other technique is the more familiar magical practice of scrying. I won't go into detail here, since you can find instructions for scrying in any decent magical text, but it's basically gazing into a dark reflective surface in order to receive images. Rather than scrying for information or divination, I used it to let my unconscious wander and open to its images. For a while I combined it with automatic drawing, keeping a pad and pencil handy and letting my hand move while I was gazing. I have also simply sat down to scry, and then picked up my sketchbook to do more fully developed drawings.
Have fun! Let me know if you have any success.