I have recently become infatuated with creating double-exposures, and they are not as difficult to make as they look! Using layers, masking, and blending, you can take anything with a strong profile and overlay it with the photo of your choice. I prefer flowers, leaves, and trees, but there are so many possibilities!
This video will show you how I created a double-exposure using ON1 Photo 10. These techniques will work similarly in Photoshop as well; most of the processing translates to a standard layer-based workflow.