Do you have an older list that you suspect has mostly decayed? Create an engaging opt-in message and send it to your old list encouraging contacts who wish to re-opt-in -- promising to remove all contacts who don't respond. Though it might seem counterintuitive to remove folks from your email lists in order to grow them, emailing only engaged contacts could improve your deliverability and increase the odds of your email getting shared with those outside your current contacts database.
Your best bet will be to give away something that’s valuable to your target market; for instance, a high-value digital asset on a niche topic. You can give away products (we’ve all seen contests where the prize is a free iPad or gift certificates), however this strategy often leads to entrants who are more interested in the money than in what you have to offer.