Lighting
Test different lighting conditions. Avoid strong lighting from behind which may accentuate the opacity of whatever is in the foreground.
To see a intereactive illustrating use of lighting click here.
Be aware of sudden increases in sunshine when a conference is taking
place, which can flood an area of the picture with strong light, again
distorting the lighting levels.
Light Compensation
Some videoconference cameras have a light compensation feature which enables some artificial tweaking of lighting levels.
These can be explored before a videoconference takes place.