Shooting against the sun, especially with low suns (early or late in the day) can produce beautiful moody shoots, with that dreamy light that is so in fashion among amateur photographer these days.
It is technically very simple, but it does take some understanding of exposure measurement and setting on your camera.
I'll keep it simple: if you tell camera to measure light on the entire image, the direct or nearly direct sunlight entering the lens is going to force the camera into exposing very little. It will think you're facing a very very bright object. The problem is: your subject's face is not in the light, and it the darkest part of your image.
What you want to do is telling your camera that however bright the ensemble is, it should meter (measure light) as it should be on the model, and not anywhere else.
In order to do so, you need to set your camera on spot metering, i.e measure light on a precise point, that you will place on the model.
It should look like that on your camera. Spot metering being the spot on the left.
Now it's pretty simple, the spot measure will be the same as your focus point (in the middle on basic camera, anywhere you focus on a more advance SLR).
Just have that spot on the model and shoot :)
FYI, it works the same on digital and film cameras.