Making Your Double Exposure
Combining multiple images into a double exposure is a great way to get unique images. This can be done with traditional film, by partially exposing 2 photos onto the same piece of film. Some modern Nikon digital cameras also allow you to do this in camera (Thanks for the tip Thad)
We are making a similar effect in Photoshop, which will give you a bit more control over how everything turns out. It also gives you the ability to combine images that were taken years apart.