Last time I talked how to keep your e-mail database clean and fresh.
A good way to get more subscribers for free is by cooperation.
You can do this by database sharing. In a nutshell:
Find companies who have a target audience which has an overlap with yours.
Contact them and explain that you want to share your database in exchange for theirs.
Create a mailing, this can be a dedicated mailing, game, competition,…
! Be certain that both companies have a link otherwise people will find it strange to receive a mailing from you for another company !
Watch how your database will get flood with new addresses.