Priority number 1 should be restraint. While having a list appears to put potential customers at one's fingertips, without restraint that list begins its slow (and sometimes not so slow) slide into degradation. What can one do to avoid this mailing list erosion? Is it possible? I believe it is... While not everyone will read everything every time, by one's use of restraint the opt in reader.
Will stick around because when they do read, they are receiving what they asked for - information, resources, tips, help, etc. They'll even read the specials. But how can you make sure your list retains its value? Consider dividing your mailing list list into "demographics" or "subscriber interests." Now stop groaning... it's really not that bad and can be a great.
Boost to your potential sales and a real plus in the eyes of your readers. Since I am a publisher, I'll use the ezine or newsletter as an example. Subscribers join to receive one's ezine daily, weekly, biweekly, monthly, etc. Right up front, by the very mailing list act of subscribing, that person knows what to expect. They receive it, hopefully read it, respond to it - a publisher's dream subscriber.