In the long term, quality is the best strategy. If you provide readers with interesting and relevant content, they will recommend your newsletter to friends and family. Another option is offering vouchers, discount codes, or exclusive content (e.g. white papers or ebooks) to new subscribers. This provides an initial incentive for registration, while the appropriate content keeps the recipients interested. Whatever you do, don’t consider buying email addresses. This is a no-go from a legal point of view.