If you venture into any marketing forum, you will see this question posed by many stressed out people who are obsessed with their stats and forget that each number is a person with freewill, their own concerns and their own reason for cancelling a subscription not just because the author is rubbish.
The bottom line in subscriber management is forget about the overall number of subscribers; as long as the trend is upwards then you are doing fine.
You do not need to keep them, you are not a prison warder and you do not want to trap people into your system.
Make it easy for them to leave and they are more likely to join in the first place and even perhaps have second thoughts about leaving.
This does not mean that you should sit back and let fate do its thing with your future, you are your own boss now and the future is under your management, if not under your control.
Remember the reason for building the list was to have a hungry audience that enjoys hearing your angle on things and you know they will at least take a look at the offers presented to them.
The list is the basis of your online success, if you can manage a list then you have a targeted audience that you take along on your journey and share in your success.
When you are invited to take part in a joint venture, then how great is it knowing that you can take your online family with you and be certain that they will lap up the venture and its benefits.
Keep engaged with your readers and talk to them on a regular basis.
Mix up your messages with advice, tales of woe and joy and then hit them with an offer every now and again.
If you can get them to open your email and force them to stop skimming and read your message, then you have just reaffirmed your relationship with them.
I get loads of emails with offers and it's a process of skimming the ones from people I trust and sometimes taking the link, then there are others that draw me in.
I love these people as they often change my whole outlook on the entire day.
This is how you keep them subscribed.