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.
Once your list is targeted, you need to spend an equally large portion of time coming up with a great deal – even if it means you might lose a bit of money on it. The underlying goal of any marketing campaign is to gain new customers, and it’s worth it to significantly reduce your profit margins to gain said customers. Once you have a surgically-honed list and an amazing offer, then you can spend some time on the design, copy, delivery methods, postage rates, date of delivery, size of the mailer…there are a lot of other options to consider, but following the 40/40/20 rule you can see how important audience and offer truly are.