Include the user's e-mail address
To unsubscribe, a user has to provide his e-mail address. A lot of users have many different e-mail addresses and often they don't remember which one they've used on which site. Often, newsletters and mailing lists use a send list that doesn't show all the addresses the mail was sent to either. Make sure a user always knows to which address your mail was sent by either showing his address in the recipient field or mentioning it explicitly at the end of the mail message like 6minutes does.

Unhappy subscribers 
Some sites are so keen to hold on to their subscribers that they don't mention the possibility to unsubscribe in their newsletter. Users who no longer want to receive the mailing have to visit the web site and figure out for themselves where and how they can unsubscribe. Although making it hard to unsubscribe has the undeniable effect that less people unsubscribe, it also has as a result that, while you may have a lot of subscribers, a lot of them don't really want to be. More "customers", but not exactly happy customers. Make sure your subscribers are your subscribers by choice, not by force. Nothing hurts your company more than unsatisfied customers.

One click? 
Unsubscription has to be easy but not too easy. Some newsletters offer users the possibility to unsubscribe with one click on an 'Unsubscribe' link at the bottom of the newsletter. Although that sounds very handy, it's not quite ideal. Users who accidentally click the link have to subscribe all over again. That takes time and a lot of users will choose not to correct their accidental unsubscription, which means you lose a user.

Unsubscribe link in every newsletter
There are a lot of ways to make it easy for users to unsubscribe. One is by including a link to the 'Unsubscribe' page on your site in every newsletter. Ideally, the content part of this page consists of no more than a short introductory text, a fill-out field and an 'Unsubscribe' button. Unsubscribing should be the only thing a users can do on the page. Don't use one and the same page for both subscribing and unsubscribing; that only confuses users. Don't ask for a password either; a lot of users are very bad at remembering passwords. A second option is by providing an e-mail address at the bottom of every newsletter and users can then send a mail with an unsubscribe message in the subject field. Don't put the unsubscribe message in the subject field yourself but leave it up to the user to fill it out. That way, you avoid that users unsubscribe by accident.

Confirmation 
No matter how you let a user unsubscribe, always send a confirmation mail to let him know he has successfully unsubscribed and be sure to mention his subscription address. Thank him for having been a subscriber in the first place and let him know he's always welcome to subscribe again in the future. Offer a link to your subscription page to show that you mean it. That way, the relationship between you and the user ends in a positive way.

Make unsubscription easy by letting users know with which e-mail address they've subscribed to your service and offer a link to your unsubscription page or e-mail address in every newsletter.

Els Aerts & Karl Gilis