Flash 9 was released a few days ago and Debian has packages to install it! But, it, at least for me, hasn’t made it’s way out to the repositories. You can download the package using wget:
Then install it using dpkg either as root or using sudo:
sudo dpkg -i flashplugin-nonfree_188.8.131.52.1_i386.deb
Also, make sure that Flashplayer 8 isn’t installed:
sudo apt-get remove --purge flashplayer-mozilla
This should work for any Debian Testing or Unstable system.