Well, its not always the C920 but poor room lighting. Ideally, you want to record/stream in a small room with the lighting to always be the same, no matter what time of day. Cover up the light sources coming in from the windows and make everything dark as you can.
Then get two 600W photographer umbrella lights and place them to the left and to the right about 6 feet away facing towards you. You can get these for under $60 on Amazon. That way, you will always maintain a consistent level of lighting and not have to fuss with camera settings.
