A while back I wrote about a few simple things retailers can do to help build up their email databases. Though building a list is important, I think it's even more important to maintain the list that you already have.
We can talk about subject lines or what time of day is the best time to send out an email until we are blue in the face. The bottom line is this: you will decrease opt-outs and encourage future opens if you produce COMPELLING CONTENT.
It might be information or a deal that is just too good to resist-- that doesn't matter. The reader just needs to feel like opening your email wasn't a gross waste of his or her time.