Photodan wrote:
...Make sure there is something of pure white in the scene, white shirt for example...
Overall, good advise, however, a white shirt is not necessarily a good target. While it is close to white, most white fabric and paper has been treated with the optical brighteners I mentioned before. When exposed to UV light, it can glow a blue/purple, and sunlight and flash can have UV in it, though we filter as much UV as practical in our flash tubes.
A good, purpose made color target is the key. We use the Whi-Bal cards, which are really inexpensive, but color neutral. The Kodak gray cards of yore are middle tone, but not necessarily color neutral (they are exposure cards, and a slight color cast is irrelevant in exposure). There are a wide variety of color targets on the market, some great, some junk. The Color Checker not only provides a neutral target, it can also help correct color bias in your camera/lens that is not compensated for with a color temperature correction.