Small list of stuff I've discovered in out of the way places about Debian. Thanks to Wichert Akkerman for being my Debian helpdesk :-)

• Trouble logging in remotely? By default debian is setup to be 'PARANOID' about forward and reverse mismatches of hosts trying to connect. Remove the 'PARANOID' line from /etc/hosts.deny
• Have modified a package and don't want apt to update it? You can 'hold' a package by doing this: echo packagename hold | dpkg --set-selections
• To run a command at each and every boot, place an executable script in /etc/rc.boot. Don't forget to remove any ~ backup files, they may be ran too
• Want fsck to fix each and every error and never wait for you to press 'y'? Edit /etc/default/rcS - there are some interesting other flags there too
• If you have a complicated network with asymmetric routes and some interfaces are mysteriously not seeing traffic, it often helps to turn off the reverse path filter. Do so by editing /etc/network/options and setting spoof_protect to no
• If you run a self compiled mailer, apt will deinstall all mail tools because it believes there is no mailer for them to work with. apt-get install equivs, and use the provided templates to create a package that convinces apt that you do have a mailer.
• If you run testing or unstable, you may find that there are packages available which don't appear in /var/lib/dpkg/available. This can be fixed by running dselect update every once in a while, which exports all available packages to /var/lib/dpkg/available again. Alternatively, just run dselect update instead of running apt-get update.
• Sometimes a package leaves traces around that can cause problems when reinstalling. Use dpkg -P packagename to purge all traces. This happens a lot to postfix. If that fails, remove the offending line from /var/lib/dpkg/info/packagename.prerm
• When moving a system to a situation where it boots from a raid device (/dev/md0 for example) and you use Debian kernel packages, you may need to edit the mkinitrd configuration so that it starts your raid. See /etc/mkinitrd/modules for details, especially the 'root=' option.
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• Also see this link from Linuxmafia