I want to upgrade my Debian lenny to squeeze but as I'm doing it for the first time I was thinking how to create the backup mentioned in the release notes:
The main things you'll want to back up are the contents of /etc, /var/lib/dpkg, /var/lib/apt/extended_states and the output of dpkg --get-selections "*" (the quotes are important). If you use aptitude to manage packages on your system, you will also want to back up /var/lib/aptitude/pkgstates.