Building a digital life is much like sending an astronaut to the moon; there is always some left over debris that is hanging around in orbit. As societal interaction shifts further online there is constant digital debris from old social networking profiles, never used email addresses, and accounts on that not-so-great Web 2.0 service that clutter the experience. There are several steps you can take to help manage your digital life and keep the debris to a minimum. The digital landscape is complicated enough without falling into information overload.
1) Name Control: The first is to own your domain name (e.g. johnsmith.com). This allows you to own your email address and also maintain your own brand online. As you change blogging services, social networks, book cataloging sites, and other such services you domain name will remain the same. There a myriad amount of domain name registration services available. (I like Hover as they provide domain privacy in their basic registration package.)
2) Email: Nowadays most people have more than one email address; a personal address and one provided by their workplace at the very minimum. However, there are also old addresses that have since been abandoned or addresses you're only have access to for a limited time (i.e. school address). In either case I would recommend forwarding the email to an active account. ( In relation to the first point several major email providers like Yahoo and Google allow users to set-up your domain name with their service.)
3) Be Selective: Web companies are always developing new services, updating old services, or pivoting old products to target a new market. Maintaining a presence or account on everything can be tiring and can create an information overload. It is best to use services that give you the most return on your investment of time (remember time is not an infinite resource). There are services (like Posterous) which aggregate content across several different services making it easier to post content.
4)Delete: Once a service has become unusable or no longer worth the effort delete your account.
Lastly, there is always the scorched earth policy, which is to disengage with the digital world entirely: delete all your profiles, log-off email forever, and give up your digital habit. However, while you no longer have to deal with the online world it also removes you entirely from the conversation. It is better to engage on your own limited terms than not at all. The world is only becoming more interconnected.