I have Debian Lenny (Testing) running on one of my partitions, and use it as my default O.S.
For some packages I use the unstable (Sid) repository , simply to get the latest available version for Debian.
To do this I have added the Sid repositories to my sources.list and created an apt.conf to make sure that the Testing (Lenny) repository is set as default.
nano -w /etc/apt/apt.conf
As root key in this line:
Of course you can also use another editor if you dont like nano.
To install a package from the unstable repository run
apt-get -t unstable install some-package
Of course you can also do this with Debian (Stable) Etch > Debian (Testing)Lenny
Resources: Debian.org, apt-howto.
Warning! The `unstable' version of Debian is the version to which the newest versions of Debian packages are uploaded first. This distribution sees all of the changes that packages go through, both small ones and more drastic ones which affect many packages or the whole system. For this reason, this version of the distribution should not be used by inexperienced users or by those who need proven stability.
For servers and other production systems stable should always be used.