Encourage people who know you and your quality to tell other people about you. One good thing to do is to add an e-mail to a friend option in every mailing list one of your sites or news letter, this plus the opt-in function will build your list higher than you'll imagine. And this makes sure that the people, who find information about you, are people who are actually interested in what you do.
To improve the e-mail to a friend option, make it big, flashy and highlighted. Another way is to take advantage of current events, and activities, give mailing list out information sheets on events that are related to what you do, more awareness greater list building, and remember, tell them to forward it to friends. Make informative and Interesting Newsletters Most members on your list are probably those who were just interested and not really active in you cause.
Try to give them newsletters that interest them or may tickle their fancy, this will make them more interested in what you have to say, and make them read more of the newsletter. When the time comes when an important announcement mailing list is to be sent, they're sure to read it and not let go straight to the trash. These are just tips on starting up, and probably maintaining the quality and size of your list, but remember stagnancy is never far.