Create a new repository and create a branch within it:
$ bzr init-repo maas
Shared repository with trees (format: 2a)
Location:
shared repository: maas
$ cd maas
$ bzr branch lp:maas trunk
Branched 5593 revisions.
It doesn't matter what you call that branch, or what branch you start with. The essence of why this is useful is because the history of every branch created within the repository directory is shared with all the other branches. This means that starting a new branch is quick — just a copy of the working tree, with a little additional disk space used for branch history:
$ du -sh .bzr */.bzr
116M .bzr
604K trunk/.bzr
$ bzr branch trunk new-feature
Branched 5593 revisions.
$ du -sh .bzr */.bzr
116M .bzr
604K new-feature/.bzr
604K trunk/.bzr
If you have a number of branches already, you'll want to migrate.
For example:
$ du -sh .bzr */.bzr
du: cannot access '.bzr': No such file or directory
117M old-feature/.bzr
117M trunk/.bzr
Create the repository in the parent directory, then reconfigure the branches:
$ bzr init-repo .
Shared repository with trees (format: 2a)
Location:
shared repository: .
$ bzr reconfigure trunk --use-shared
$ bzr reconfigure old-feature --use-shared
Repeat that for the rest of your branches. The first reconfigure will take some time but the second and subsequent will be quick.
That's it.
$ du -sh .bzr */.bzr
116M .bzr
604K old-feature/.bzr
604K trunk/.bzr
In other respects these branches / working-trees behave the same as their standalone cousins.