Work with natural light whenever possible. The gentle hues of early morning or late afternoon are excellent for creating softer tones and shadows.
Use reflective materials. A large Styrofoam sheet or piece of white poster board can be held up by a production assistant to reflect light back at the subject and soften harsh shadows.
Place a UV filter on your camera to reduce glare and haze.
Take light readings with a light meter before you start shooting. Setting up the scene before you shoot saves you frustration and multiple takes due to lighting problems.
Snap test shoots to design a lighting blueprint for the duration of the shoot. From the test shoot, you know exactly when to shoot in which locations and what supplies you will need.
Focus on using light to tell a story. In a film, adequate lighting is not enough to convey a mood or connect characters to an audience. Think of the light in a shot as another character interacting with the scene.
Take a photography class. The basics of lighting photos and film are similar, and developing a deep understanding of photography lighting will improve your eye for spotting it in film.