We've already mentioned that clouds and trees can be natural light diffusers, but don't be afraid to come up with your own methods for diffusing light, or reflecting it gently where it's needed.
Creating a diffuser or a reflector is actually pretty easy. If you're inside and have only one harsh light source, like a lamp, simply put a piece of paper or a light cloth over it. If you're outside, a light-colored umbrella positioned between your subject and the sun can diffuse the light -- just make sure the umbrella itself isn't in your photograph's frame.
Reflectors are a good way to reduce shadows and improve light in a photograph. If you've ever seen a professional photo shoot, you may have seen the photographer or the photographer's assistant using white or metallic cloth stretched over a frame to reflect light onto the subject. You can do the same thing. A white tablecloth can reflect light onto a subject (this is a great tip if you're taking photos by candlelight). If you're outside, a white cooler lid placed below your subject can throw light upward. You can also reflect light by wrapping tinfoil around some cardboard. Play around with different methods and materials until you find some that work for you. As an added bonus, you'll have them in your photographer's tool kit, ready for the next time when the lighting is less than ideal.