Good exposures ETTL and 550 EX 420 EX

I'd be grateful for any tips anyone might have as to how to get consistent good exposures shooting portraits with a 550 EX and a couple of 420 EX slaves. For instance a young girl with dark complexion and dark hair. I get a good shot and then she puts on a white hat and the skin and hair become underexosed, even though the focus point remains on the eye. I'm using a Rebel XT for a camera.

what is your metering mode? try spot metering? Using a light meter would help. Expose for the face.

WElcome to POTN!!!

You need to use FEC to control the flash exposure since ETTL-II is going to determine expsoure based on the reflectivity of the subject and not where the AF point is. So, depending on the colors worn by the subject, you exposure may vary even with the camera settings and focus point remaining the same.

To get good exposures from the get-go without needing to mess with FEC, you will need to set your flashes to M mode and use a flash meter to determine the aperture setting you'll need.

BTW, your camera metering modes will have no effect on your flash exposure because they are used only for ambient light metering.