Ask the subject to take off her dress. Then she will become overexposed, but not the white top. :bsmilie:
Seriously... for portraits, a good place to meter is the face. If you know the perculiarity of your camera's meter, you will know what meter value to expect for each skin tone with some experience. For example, when I meter my daughter, who is a pale Chinese baby, I will expect a reading of +1/3 or +2/3 in Manual mode. If I get that right, the exposure is usually good. Shooting RAW helps a lot because you get about 1 stop of play in post processing. If the face is correctly exposed, but the details of the dress is muted because of too much light, then you will have to use Photoshop to help you. A quick and dirty way is to use the burn tool and burn the highlights.
Another very useful way is to apply the blue channel as another layer and choose 'luminosity' as the blend mode... Then use layer mask to apply this only to the dress. But this is quite 'cheem' and not for everyone... but if you are interested, read the book 'Skin' by Lee Varis.