This basic list developing formula can be summed up in 2 words: ad swap.
Normally, you trade ads with other eZine publishers, ideally of the very same mailing list size or bigger. You relay the eZine publisher's ad to your mailing list while the eZine publisher supporteds your ad to his list. Yes, you are really cross backing or cross promoting each others provide to each of your newsletter.
Your ad's goal must be to obtain as many subscribers possible from the other eZine publisher's newsletter to enroll in yours.
The outcome: you grow your subscriber list. This approach does not require cash, meanings it can be done for free. And the return of customers? Infinite!
You only have to do this with one eZine publisher a minimum of once, because his customers who are also your subscribers can now be followed up within the limits of your subscriber list.
Do ad swaps with as many eZine publishers as possible and quickly, you will have a big subscriber list of your own-- built complimentary.