2. Set clear expectations around how quickly you process their unsubscribe request.
This is a corolloary to #1, but it's important enough to call out separately. Ideally you subscriber will not get anymore email after they unsubscribe. But if that isn't possible you must tell them exactly how much more email they can expect and in what timeframe. It doesn't guarantee that they won't complain about subsequent messages, but it helps.

3. Use the List-Unsubscribe mechanism within the header of each message.
Many ISPs are considering adding an unsubscribe button, either instead of or in addition to the "this is spam" button. This mechanism allows that to happen for your messages. Instructions on how to do that can be found here:

4. Your "from" address should make sense to actual humans.
This, by the way, applies to all parts of your "from" – display, domain, and so on. Making sure your "from" is clear increases recognition of the message and helps recipients and ISPs alike identify the good from the bad.

5. Don't just keep re-sending your message indefinitely.
When ISPs have outages the continual re-sending from marketers makes it even harder for them to get back on line. It's best to investigate the matter and follow specific re-try protocols when available. But MAAWG recommends, in all cases, discontinuing re-tries after four days to accommodate weekend outages.

6. Pay attention to the bounce messages.
Errors that tell you that "delivery is not authorized" indicate that you've violated acceptable use policies. In layman's terms, they think you are spamming them. Continuing to send email to that domain is going to make matters worse. Investigate the issue to figure out why you are being blocked.

Source: Return Path