Posts change, videos disappear, tweets are deleted. Links rot:
http://gigaom.com/2012/09/19/the-disappearing-web-information-decay-is-eating-away-our-history/

There are tools to combat this. Please post your own tips and techniques.

General page archiving: https://addons.mozilla.org/en-us/firefox/addon/shelve/
Shelve can be configured to automatically save every page you visit. Each version is saved in a timestamped folder under the domain name (in your Firefox profile folder).
Recommended file name template: %{shelvedir}%/%B%/%Y-%M-%D-%h-%m-%s-%l%/%[ctF]%e

Also see: http://webcitation.org

Youtube Archiving:
Youtube-dl: http://rg3.github.io/youtube-dl/
Recommended command line:
youtube-dl --verbose --write-description --write-info-json --output "%(uploader)s__%(uploader_id)s__%(upload_date)s__%(title)s__%(extractor)s__%(id)s.%(ext)s" [URL]