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:

wget http://http.us.debian.org/debian/pool/contrib/f/flashplugin-nonfree/flashplugin-nonfree_9.0.31.0.1_i386.deb

Then install it using dpkg either as root or using sudo:

sudo dpkg -i  flashplugin-nonfree_9.0.31.0.1_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.