I run lenny on my home server. That said, there are times when I want new packages which are not available on lenny backports. One solution is apt pinning (another is compile from source :p).
First, add the unstable repo entry in /etc/apt/source.list:
deb http://ftp.us.debian.org/debian unstable main
Then, add the following in /etc/apt/preferences:
Package: * Pin: release a=unstable Pin-Priority: 800
While this works generally, please refer to apt_preferences manpage for more details. Finally, run
apt-get update && apt-get install -t unstable <your-new-package>
to update and install the package from unstable. Depending on individual requirements, you may choose to substitute testing for unstable. Enjoy!