My hat (base-ball cap style, or on cold days my wool cap) underneath, since wearing it under the dark cloth is a hassle anyway. I actually use first the method David mentioned, except for when I have the bellows stretched out too far to use it.
I was having the opposite trouble -- the bellows blocking the light when very compressed - a clothes pin on the top of the bellows which kept the bellows from sucking into the back of the camera worked nicely.
Vaughn