Motivate customers/recipients to sign up for your marketing newsletter

Making visitors of your Web site sign up for your marketing newsletter is getting increasingly difficult as people are more reluctant to give away their email address (and they're perfectly right facing all that spam). Bonus can motivate people/recipients to sign up for more marketing newsletter. For example, you could offer a white paper or - preferably - a detailed case study for all those who subscribe to your marketing newsletter, or new subscribers get a special tips and tricks edition of the newsletter, or a funny Winamp skin. The possibilities are endless, but make sure the bonus is the right bonus for the right people. Whatever you offer, it must be directly related to the newsletter. The motivation to sign up for the newsletter is the same as the motivation for getting the bonus, and the bonus acts as a multiplier, not as a separate motivation.