I've never had the flow of promotion missions stop, playing since late december and I'm a Count now. I think this is because my playstyle is a little bit of everything. I travel a lot, do a lot of odd jobs for any and all imperial factions, got to allied status with the Empire as a whole, and have plenty of kills under my belt against pirates and the Empire's competitors. My progression ranks are Expert/Dealer/Pathfinder, so I guess if you're at that point you should be ranked up as high as I am.
So yeah, move around a lot and get allied with enough minor factions that the major faction starts to notice you. Take plenty of pirate sweep and assassination missions (but make sure you know what faction you're targeting so you don't get -rep where you don't want it), do some trading and exploring and sell at imperial stations, and if you just keep playing normally long enough you'll notice the offers being thrown at you left and right. Worked for me anyway.