Perhaps the most common mistake I see during location shoots is photographing the model in harsh light. On a bright, cloudless day, it is always a good idea find a shady spot where the light is soft and diffuse. The light on the model can be greatly enhanced by reflecting some of that light back onto the model using a handheld circular reflector, which also softens the reflected light.
Such was the case in the old part of Bisbee, Arizona, during a shoot with Tiffany Felisha. She posed in a shady area along some store fronts. My great friend Benjamin James stood in the sunlight and reflected light back into Tiffany using a circular reflector. Not only does this fill in the shadows a bit, but it produces nice catchlights in the eyes:
Reflectors are inexpensive, collapsible, and are available in different colors. The most useful colors are gold (for a warm light), silver (for a lot of light) and white (for a softer, more muted light).