pipit:~# apt-cache policy
Package Files:
 100 /var/lib/dpkg/status
     release a=now
 500 http://packages.debianbase.de ./ Packages
     origin packages.debianbase.de
 500 http://packages.debianbase.de ./ Packages
     origin packages.debianbase.de
 200 http://www.backports.org sarge-backports/contrib Packages
     release o=Backports.org archive,a=sarge-backports,l=Backports.org archive,c=contrib
     origin www.backports.org
 200 http://www.backports.org sarge-backports/main Packages
     release o=Backports.org archive,a=sarge-backports,l=Backports.org archive,c=main
     origin www.backports.org
 500 http://security.debian.org stable/updates/contrib Packages
     release v=3.1,o=Debian,a=stable,l=Debian-Security,c=updates/contrib
     origin security.debian.org
 500 http://security.debian.org stable/updates/main Packages
     release v=3.1,o=Debian,a=stable,l=Debian-Security,c=updates/main
     origin security.debian.org
 500 ftp://ftp.fr.debian.org stable/contrib Packages
     release v=3.1r3,o=Debian,a=stable,l=Debian,c=contrib
     origin ftp.fr.debian.org
 500 ftp://ftp.fr.debian.org stable/main Packages
     release v=3.1r3,o=Debian,a=stable,l=Debian,c=main
     origin ftp.fr.debian.org
 500 ftp://ftp.u-strasbg.fr stable/contrib Packages
     release v=3.1r3,o=Debian,a=stable,l=Debian,c=contrib
     origin ftp.u-strasbg.fr
 500 ftp://ftp.u-strasbg.fr stable/main Packages
     release v=3.1r3,o=Debian,a=stable,l=Debian,c=main
     origin ftp.u-strasbg.fr
Pinned Packages:
     xserver-xfree86 -> 4.3.0.dfsg.1-14sarge2
     mysql-server -> 4.0.24-10sarge2


pipit:~# cat /etc/apt/preferences  
Package: xserver-xfree86
Pin: version 4.3.0*
Pin-Priority: 1001

Package: mysql-server
Pin: release a=stable
Pin-Priority: 1001

Package: k*
Pin: release a=sarge-backports
Pin-Priority: 999

Package: *
Pin: release a=sarge-backports
Pin-Priority: 200