Debian testing or unstable

Debian unstable and testing are usually fairly up to date, so this should be enough:

sudo apt install git-annex

Standalone backports for all releases

If the version shipped with Debian is too old, the NeuroDebian team provides a standalone build package that is regularly updated and that should work across all releases of Debian.

Debian 8.0 "jessie"

sudo apt install git-annex

There is also a backport for jessie, even though it is often out of date.

Follow the instructions to enable backports.

sudo apt -t jessie-backports install git-annex

Debian 7.0 "wheezy":

sudo apt-get install git-annex

Note: This version does not include support for the assistant. A backport is available with the assistant and other new features.

Follow the instructions to enable backports.

sudo apt-get -t wheezy-backports install git-annex

Debian 6.0 "squeeze"

Follow the instructions to enable backports.

sudo apt-get -t squeeze-backports install git-annex