Debian: Lenny Default with Sid Repository

First add the sid repository to your '/etc/apt/sources.list'. See Below

# sid
deb http://http.us.debian.org/debian/ sid main contrib non-free
deb-src http://http.us.debian.org/debian/ sid main contrib non-free

Now create a new file called 'apt.conf' in '/etc/apt/'. Add the following to it's contents

APT::Default-Release "testing";

Now update your repository. If by any chance it appears that the cache limit of apt is not enough simply add the following line to the 'apt.conf' file you just created and update your repository.

APT::Cache-Limit "20000000";

To update your apt repository simply type the following command into a terminal:

# apt-get update