/etc/apt$ cat sources.list
deb      http://archive.debian.org/debian/              lenny           main contrib non-free
deb-src  http://archive.debian.org/debian/              lenny           main contrib non-free

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

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

#deb     http://archive.debian.org/backports.org/       lenny-backports main contrib non-free
#deb     http://archive.debian.org/backports.org/       lenny-backports-sloppy main contrib non-free

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

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

/etc/apt$ cat preferences
Package: *
Pin: release a=lenny
Pin-Priority: 700

Package: *
Pin: release a=squeeze
Pin-Priority: 650

/etc/apt$