Here are my top tips:
1. Mail regularly. Too many people are afraid to mail their lists because they're worried that they'll just end up with annoyed subscribers. If you're sending information to your subscribers that can actually help them out, they stick around.
2. Accept the fact that people are going to unsubscribe. Your offers aren't right for every person that joins your list and that's okay. Unsubscribers aren't a bad thing, they actually help you to make your list more targeted and improve your conversion rates.
3. Keep content ready to go. Don't wait until your list is running out of new emails to receive from you before you create more content. You should always be thinking of new content ideas in advance so that there is always something to read.