/etc/apt/sources.list  ….

deb http://ftp.us.debian.org/debian/ lenny main contrib non-free
deb-src http://ftp.us.debian.org/debian/ lenny main contrib non-free

deb http://security.debian.org/ lenny/updates main contrib non-free
deb-src http://security.debian.org/ lenny/updates main contrib non-free

deb http://volatile.debian.org/debian-volatile lenny/volatile main contrib non-free
deb-src http://volatile.debian.org/debian-volatile lenny/volatile main contrib non-free

update 8/22/10: i just recently discovered how easy it is to update from debian stable to testing…simply edit this file and replace lenny with whatever is currently the testing release name (currently squeeze). i’ve always thought apt-get was pretty cool, but now i respect it even more.