In ubuntu we now have a bzr plugin which makes building packages maintained in a bzr branch easier. This page describes how to use it. For a more general overview, see the documentation for Distributed Development.
The plugin is available in the 'bzr-builddeb' package in both Ubuntu and Debian. As well as the Distributed Development documentation linked above you can read the user manual at http://jameswestby.net/bzr/builddeb/user_manual/