I bought a book not long ago for some inspiration -- The Mandala Guidebook by Kathryn Costa. She offers a wealth of suggestions and examples to help reader-artists create original mandalas. I tried one immediately because it relates to an approach I've had in mind for a while, i.e., using magazine images with hand drawing.
This is pretty simple, but I love the vibrant color that results from using magazine images on a black background. (If you decide to buy the book, which I would suggest if you enjoy creating mandalas, see "Magazine Mandala, Style 1" on page 95 of Costa's book.)