Proper lighting is the key to realism in any composite. If the lighting of the foreground subject and the background don't match, even the cleanest composite will look "phony". When lighting your green screen shot, there are many things that can adversely effect your final composite. The key is getting it right "in the can". Following these simple guidelines should ensure a smooth Ultimatte process in post. 

1. All of your light fixtures should have the same color temperature. 
2. White balance the camera, illuminating the white card with the foreground lights. 
3. Light your foreground subject first, then your backing. 
4. The backing should have a consistent color temperature and brightness at every point on the backing. 
5. Check your backings consistency with an incident light meter or a waveform monitor. 
6. Never use a dimmer switch. 
7. Shoot a screen correction whenever possible.