Imagine my excitement when I discovered the ability to shoot multiples in camera! Anytime I can do something in my camera rather than in post, I'm all about it. If you have a camera that affords you this option, give it a try. I'll share some pointers you might want to take into consideration in this post.
Whether heading out with a plan in mind or shooting opportunistically, it's important to think about how your images go with each other. Is one image going to be the primary content? If so, envision how the second (or third, fourth... up to nine) image can complement it to achieve interesting results.
DESIGN & PLAN
This is a critical component since we're not moving around layers in Photoshop. It's imperative to "see" the layering for proper placement. For example, in this series, I tried to keep the face of the Buddha clear which meant I had to position the flowers in such a way that the orientation of the final image was pleasing (note how the Buddha was shot vertically but rotated 90 degrees in the final image?).
The Canon 5D Mark III gives several options for processing the exposure of the final image: Additive, Average, Light, & Dark. The series above was shot in "Average" mode. In this case, the camera will adjust automatically to yield a standard final image if shooting repeat frames of the same scene.
I shot another similar pair in "Additive" mode which resulted in a lighter final image. You really need to experiment with the exposure compensation you set while actively shooting in anticipation of what the camera will return. Mark III owners can read more about this on pp 177- 183 in the owner's manual which is posted on the Planet 5D bog at: http://gdlp01.c-wss.com/gds/8/0300007348/01/eos5dmkiii-im-c-en.pdf
Carry around a CF card of possibilities. You have the option of adding multiple images to a single image already on the card. Why not carry around a small CF card with possibilities in need of a companion shot?
ABOVE ALL, HAVE FUN & CREATE ART!