There was a pretty good discussion on this topic not that long ago. Search the forum for the phrase "What am I doing wrong" and you'll find it. Sorry, but I'm not sure how to link to another forum post inside a message. Maybe somebody can help us out with that too.
Basically the effect you're looking for is most easily accomplished on white seamless, with the background lit about a stop over what the main light meters at. The background will "blow out", appearing perfectly white. Generally, if the light bounces back on the model it's considered a mistake, as the background is either too heavily lit, or the model is too close to it. However, this "mistake" is sometimes used as an interesting effect.
Hope that helps.