This behind-the-scenes video snippet shows how I used modeling clay to visualize the lighting for a painting of a snake attacking the nest of baby sauropods. (Link to YouTube)
I based the stone figures of Ebulon (Dinotopia: Journey to Chandara) on a modeling clay maquette as well.
These maquettes take only minutes to make, but they provide a wealth of information. You can turn them to any angle or put them into any real lighting environment. Note for example how the warm light bounces around in the little shadows on the lit side.
I recommend using clay with a light gray or cream tone, which photographs well while allowing you to see the qualities of light and shadow.
After you're finished with a project, you can smoosh it together and use it again on the next job. It's cheaper than oven-hardening polymer clays like Sculpey. Modeling clay is non-toxic and safe for kids.
It is is available from several manufacturers, using closely related trade names:
Plastalina(various neutral tones)
Modeling clay (light gray)
You can also get Air-Dry modeling clay, but it won't be reusable after it dries and hardens.
More about making maquettes on my video "How I Paint Dinosaurs"