In this contest you offer a sizable prize that people want. For example, a while collection of a product.
Winning is a draw, but you allow people to have multiple entries.
A participant can share the contest and every share gives them another entry.
That way they can share with many friends and have many entries = many more chances to win.
Outgoing participants with large networks will be attracted to the contest because they could have a better chance of winning than others.
Every tie they share the contest you collect more emails for future email campaigns.