There is certainly quite a lot that you can accomplish with natural light, but you're at the mercy of the weather outside, and sometimes, adding a bit of flash can go a long way to augment the light at hand or to even reshape it entirely. This great video will show you some different ways to use a flash to create a variety of looks when you're shooting portraits outdoors.
Coming to you from Ed Verosky, this helpful video will show you how to use artificial light in tandem with natural light for outdoor portraits. The important thing to note is that Verosky is not using an ultra-expensive lighting kit here; in fact, he's simply using a speedlight and a softbox. By remembering the fundamentals of light (bigger and closer sources produce softer light and vice versa), he's able to achieve a wide variety of different looks. Because it's an overcast day, the speedlight doesn't have to compete with the ambient light as much (and Verosky keeps it relatively close to the model); if you're shooting on a sunny day, you might want to consider a modifier with a dual-speedlight bracket or a dedicated monolight. Check out the video above for Verosky's tips.