Lars Wirzenius have given a lecture in Debian packaging and posted the presentation on his blog for our learning pleasure.
The presentation is in .pdf here
I've now given this tutorial twice: once for three of my friends privately, and once for a larger group. It only covers the very basics of creating packages, and there's much more to learn. Some day I might do a second part, and cover things like maintainer scripts, advanced dependencies, debhelper, pbuilder, and piuparts (the last three were only briefly touched upon today).