Before you start taking your real shots, take some sample pictures to test your lighting.

Pay attention to:

How "white" your whites are

Shadows and reflections

Under- and overexposure

Auto-white-balance sometimes doesn't function properly. If you're indoors, use the tungsten or incandescent (3000K range) setting. If you're outdoors, select daylight (5600K). On some cameras you can also automatically set white balance by taking a picture of a sheet of paper.

When you take pictures on a light background, the picture will sometimes appear too dark. To compensate for this, increase the exposure.

Once you have a lighting setup that you're happy with, leave it the same for the entire shoot.