They may not be your direct competitors but businesses that complement yours. You can all agree to send offers to your individual lists with a link to your mailing list lead capture pages. With enough businesses joining forces, you could amass of very large list in a short period of time. You would not be violating any one's privacy using this method because there is no exchange of lists in the process.
Each business sends out a recommendation to their own list of prospects and customers with an attractive offer from one of their coop partners. This makes generating leads less costly and offers better results. When this method is done tactfully and upfront mailing list with an attractive offer, those with an interest in that particular subject are more than happy to take the freebie.
It isn't necessary to build a large list; it is possible to work effectively with just a list of 100 people. I've utilize the power of postcards to mailing list great effect. Pick 100 and likely prospects from your list and send them a postcard each month for over two weeks. You can do the same thing by email with an e-card that has inspirational quotes or incredibly funny cartoons.