There have been a lot of posts lately about people having problems installing Flash, the following both work with no issues that I have seen.
If you run Stable/Etch you will want to use backport.org
Use whatever text editor you prefer as root add the backports.org repository to your /etc/apt/sources.list. I run KDE and prefer kedit so I would do kdesu kedit /etc/apt/sources.list in konsole.
[quote] 1. Add this line
deb http://www.backports.org/debian etch-backports main contrib non-free
to your /etc/apt/sources.list.
2. Run apt-get update
3. All backports are deactivated by default. If you want to install something from backports run:
apt-get -t etch-backports install “packageâ€