Because I see a lot of very complex tutorials, I’d like to point out that it can be done much easier:

  1. add deb http://ftp.nl.debian.org/debian experimental main contrib to your /etc/apt/sources.list (replace nl by your favorite mirror)
  2. aptitude update
  3. aptitude -t experimental install flashplugin-nonfree
  4. done :)

That’s all.