You can use
GetResponse or
AWeber to take care of this; what you can also do, is charge for your newsletter, or create some back-end products to sell, or both!
Typically, if you supply good enough information then people won't mind paying for it, and/or getting pitched every now and then (especially if they're past clients).
Probably wouldn't be a bad idea to get a book or two from Amazon on the subject also.
Colm