The last thing any of us want is for someone to unsubscribe but even if we get everything right, sometimes people's priorities change and they just don't want our emails anymore.
Obviously there are some common reasons that people opt-out that are our fault, eg: poor creative, bad subject lines, un-remarkable content, frequency to high etc. etc.
Some of these can be handled before sending the emails by just spending a little more time empathising with the audience and focussing on why you are actually emailing them in the first place and then ironing out your priorities.
Once someone who has the email decides they want to optout, obviously we make it easy: click a link submit a form. That does not mean that your relationship is over, it just means that from their most recent experience they do not want those emails anymore.
What ever you do with the opt-out form, avoid asking them to enter their email address, that scares people. If people think that anyone can enter any email address and press go to opt it out, they won't trust you and that can domino down to other parts of your core business.
Now this optout form can do more than just let someone optout...
Apex Auctions and Pure360 found that Apex's problem was that they were sending too many irrelevant emails as everyone was on the same list. Pure360 built them a preference centre so people could tell them what they wanted to be emailed about. Apex started sending a lot less emails, but everyone was getting emailed about the products they were interested in, retention went up, ROI went up and then subscriber acquisition went up.
I would also suggest considering collecting feedback at the that point, to elaborate on a post from EmailGarage, maybe a tiny survey or just a text area for comments. personally a survey would easier to transfer in to stats and would require less resources mind you. with the results you can easily and quickly keep an eye out to see if you are doing some thing wrong.
BlueSkyFactory blogged about an optout page that also gave you the choice to specify how many emails a week in their opt-out form. I'm not realy a fan because if you think about it how would you manage that? You send 3 emails out in one week but some people are only letting you send one - which one do you send?
I love Clickz's subscription page, they give you a tick box for everything they publish and then a tick box to manage the frequency - so I can either get an email every time a new article is published on one of my chosen catagories or I can get a weekly email with a snippet and link for everything.
Giving the recipient as much control as possible and being as up front as you can, will earn you trust and get you interactions.
Especially nowerdays when so much content is bloggy, the RSS feed can easily be used to grab content, format it into email html and email it out depending on the rules defined by the users. It can be really easy and low maintenance, I think it's great.
