Since I've been less active packaging for Gentoo recently, I've been trying to at least lurk in #gentoo-dev-help and give a helping hand with packaging. If I can't help them directly, at least I try to give them a gentle shove towards something that can.
As a result, I've been collecting various bits of documentation about how to package stuff. This post is a result of this, as well as some other digging around I did to flesh it out.
General development documentation
Channels of communication
Lots of projects and herds within Gentoo have their own mailing lists and irc channels.
Language-specific
Packages written in different languages often have their own way of dealing with packaging:
Herds
Same goes for herds.
Other stuff
These don't fall into any other categories, but are still useful.