Use a reflector to direct light back onto the subject. Properly aimed under the right conditions, a reflector can work wonders. Try it, and you’ll wonder how you ever got along without one.
Yes, you can use Fill Flash to wash away the shadows, but that can have some drawbacks. The color temperature of the flash will not match the ambient light, and mixed lighting can cause color shifts. The irises of the model’s eyes are going to reflect a white spot in the shape of the flashtube. Sometimes the model’s skin will appear shiny or even slick. Like Poison Ivy, even though these maladies can be fixed, it’s better to avoid them.
My collapsible reflector is gold color on one side and silver on the other. I bought it from B&H Photo for less than $17 but every major photo retailer sells them. It came with a fabric case and it folds up into a disk that’s about seven or eight inches in diameter. Fotodiox makes a 22-inch job that can do double duty as a diffuser, as well.