I think mythic's rolling population stuff worked well, at least Dark Crag has the best population I've ever seen -
Dark Crag Statistics: Population Data
problem is they dropped the ball with the amount of servers on pre-release (too few) and the amount a week later (too many)
As for keeping a sustained balanced population, or trying to repair a broken one, not entirely sure. Give the players a chance to "cash in" their character in exchange for an appropriately leveled/geared one on the other side?
Granted factionless is the way to go... but developers are pussies.
