I got problems to push my project to a brand new branch created on Launchpad and this helped me out.
I've already created a new branch on Launchpad and imported my SSH key to be able to use sftp instead the standard 'lp:' (which is alias to http://bazaar.launchpad.net/), because http connections doesn't support mkdirs, so it wouldn't push a project with a directories.

You must change 'myproject' to the name of your register project on Launchpad and 'launchpad_user' to the user with you've registered the project.
This last is just in case your machine user name is different to your launchpad user name. If those are the same you can take off the 'launchpad_user@' stuff between 'sftp://' and 'bazaar.launchpad.net/...'.

user@machine:~/bzr/$ cd myproject
user@machine:~/bzr/myproject$ bzr push sftp://launchpad_user@bazaar.launchpad.net/~launchpad_user/+junk/myproject --use-existing-dir