As Internet marketers we know that staying relevant to our existing subscribers is one of the most important aspects of our job, if not the most important. These existing subscribers are people who have already either expressed interest in us, or they have actually purchased from us.
You already have their attention once they subscribe or buy so it is important that you keep their attention by:

Tell subscribers what to expect — Upon subscribing to your list you should tell your new reader exactly what to expect from you, then stick to it. If your newsletter is not suppose to promote products, then don’t promote products. If your newsletter is only to promote products relevant to a certain group then only promote those products. Don’t stray from what you promised. Be very specific when you tell your subscribers what you will do by telling them in both paragraph form and bullet form, that way they will not miss anything and will not be confused.
Educate your subscribers — Believe it or not, some subscribers still do not really understand the Internet much and get confused about email lists. Therefore you should explain to them exactly what they have joined, and how to make sure they do not miss a single email by white- listing your email address.
Do not spamUse and check out all products before you recommend them to your subscribers. Unless you have used the product or personally know the creator, you cannot really know for sure if the product does what they say they will. Legally you have protected yourself from the spam police by having an opt in or you only email people who are your customers but to most subscribers, anything not in their niche is spam.
Be consistent — If you are not consistent, and regular in your emails your list will forget about you and then they will delete you upon receipt of the irregular and occasional email. To stay in their mind, be consistent. There are arguments that suggest once a week, some say monthly, regardless of what you decide, be consistent.
Create awesome headlines — Even if your subscriber is expecting to hear from you people subscribe to and get so many emails that yours can be missed. Make sure you create a headline that people want to open, give it some thought.
Personalize — Everyone loves to see their own name, and they love to feel special so personalize
your email newsletters so that they see their own name in it. You can even put their name in the subject line which they will like even more. It will make them feel closer and more trusting of you.
SegmentMake sure that you segment your subscribers at the very least into those who have bought and thoughts who have not bought. It is important because you do not want to market the same product to someone who has already purchased it. It is nicer if you can segment further all the way down your product funnel then your marketing can be more specific.