A number of handy tips pages, especially packaging.
Quick-start guide
and of course the apt-howto

For pinning or default releases, see this conversation
One that keeps popping up although I haven’t used it much… the Debian Grimoire
Debian repository HOWTO and commentary on it with hints

General dpkg/apt tips from the Debian FAQ

Creating Debian packages with Checkinstall.

Specific tips as well…
Fix an X install
dpkg-reconfigure xserver-xfree86

/etc/apt/apt.conf useful settings
APT::Cache-Limit 10000000; # for apt crashing with too much data after update
APT::Default-Release “stable”; # to target a release and yet have others available