I don't think that anyone who reads this needs to be told that you need to be careful what you put online. In today's world of search engines and other web crawling software, the internet has become the world's largest copying machine. But Google archiving isn't the only problem; take Facebook for example. If you want to delete your account from that site, be prepared for the long haul.
The main option that Facebook offers is to deactivate your account, not delete it. They keep an archive of all of your information so that, just in case you didn't really seriously truly want to delete your account, it can be restored with a minimal hassle. At least, that's one reason they keep it; who knows what else they do with the information in terms of deals with their advertising partners. So what do you have to do in order to permanently delete it? You have to go through and manually delete all of the information that you've put up there. Every photo, every comment, every crazy little application you added to your profile. Then, and only then it seems, can you delete your account permanently. (Note: This does not affect any information that any third party applications that you added may have already collected on you.)
Apparently sometimes even that doesn't work. Some other tactics that I've read to protect your information are to start flooding your profile with false things. Un-friend your actual friends. Invite people you don't even know. Change location and personal information. Enough of this and no one will be able to separate fact from fiction; it also has the benefit of looking like your account has been hacked. Another way is to violate as many of the terms of service as possible without doing anything illegal. Start spamming groups and walls and pretty much attempt to be as annoying as possible. Send emails to Facebook mocking their inability to stop you. Account deletion is sure to follow pretty quickly.
Of course, all of this may be a moot point. I think my generation is probably the last generation that will even care about this. The next generation has been raised on the internet. They are not only fully aware that social networking sites like Facebook and MySpace are storing all of this information about their personal lives, but they fully expect it. It's going to be pretty interesting in a few years when my generation is interviewing the next generation for job positions and their entire personal lives for the past ten years will be online for anyone to see.