This was taken using a 17-55mm f/2.8 IS and a single off-camera flash:


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Here's the setup shot:


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As you can see here, the view out the window was quite distracting, so I simply taped a large piece of white cloth over the window. Then I shot the flash through the white cloth thereby diffusing the light and making it look both flattering and natural. Unfortunately, you'd need a way to trigger your flash off-camera. There are cheap (yet somewhat unreliable) ways to do that (CTR-301p). Here I'm using Cybersyncs--slightly more costly but worth every penny. You can see more of the pictures I took with her here.


Else, you could use your 580EXII on-camera and bounce the flash off a white wall to diffuse the light and make it look more flattering. Might be the easiest thing to do.