Awhile back, I remember flipping through a great cinematography book called Painting with Light. Written by cinematographer John Alton, it covered a wide range of topics from lights to modifiers to techniques. Some of the equipment may be dated, but there is a wealth of knowledge in that book and I'm glad it is still available.
One item that stuck with me (and is the inspiration for this video) is Alton's "test light", a DIY device that lets the cinematographer check how light will fall on a subject's face before actual lights are brought in. It's a great time saver, and allows for simple experimentation. Watch the video to see how to make one, or rather re-purpose something you already have.