Shooting natural light can be great, but the beauty of flash is the control you have over the power and shape of the light. Sometimes though, a combination of the two is best, and this excellent tutorial will show you exactly how to effectively mix them for a dramatic portrait.
Coming to you from Gavin Hoey of Adorama TV, this great video follows him as he shoots a location portrait. The idea for the shoot is pretty great, as it involves a model wearing a long flowing dress among similarly flowing grass, but as you can see early on, the portrait with just ambient light is a bit flat. There's not necessarily anything wrong with that, and a lot of photographers love to shoot with flat, natural light, then shape it in post, but it doesn't really match the look Hoey was going for. On the other hand, completely overpowering the ambient light with the flash would leave a lot of the background unlit and kind of ruin the point of shooting the portrait on location; thus, Hoey opts for a balanced mixture of the two and gets exactly what he wants: a dramatic shot that shows off the location. Give the video above a watch to see how it's done.