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a multi-branch repository, "bzr multi-pull", periodically
(daily, hourly, whatever in needed).  This will always have
the mirror in a consistent state, but within the size of
your window, you could be missing some events.

This is clearly not a prefect mirror; I use this because
for me network operations are very "expensive", both in
terms of cost (I will quickly exceed my maximum 30-day
transfer limit if I'm grabbing lots of stuff over the
internet) and in terms of speed (round trips are
very expensive), so I'll do read-type operations against
the local mirror.  But I don't ever commit to the local
mirror; I commit to working branches and push/merge up
to the master, then sync the local mirror to get it
back up to date.

To get closer to having them in sync you'll need to set
something up that triggers off of commits.