I gave the guys at the Hutt Cartel my very simple advice for making a small idea a big faction: marketability. Tirdarius hints at it, but I'll expand.
How *popular* is your idea? Let's take the Echani for this example. Small niche even in the Star Wars universe. Cool idea, though, but most people without an express interest in Star Wars EU wouldn't know about it. So let's go with... not that popular of an idea. We can still work with this.
What have you done to make your idea marketable? Images. Images, images, images. Not only do you need to dress up your faction using the software provided, but ***MAKE YOURSELF PRESENTABLE***. The less popular your idea is, the more pleasing to the eye everything that involves your faction needs to be. Your avatar, your role-plays, take it to the next level.
Are you being persistent? Now, this doesn't mean being annoying, but if I were a faction leader with a small faction, I'd be PMing every staff member once a week: "Hey, so I just wanted to hit you up, what options are available to small factions to be able to boost our membership?" Or I'd be leading topics like this very one. Or I'd be PMing the board owner. "Hey, so is there a possibility of my faction getting a Rank Title? I noticed you've given them out to other factions and respect the fact that you told me last week you only give out Faction Rank Titles to "established" factions, but I think you should re-evaluate your position concerning my faction."
Are you greeting new members? If your signature doesn't have a link to your new faction, you've already messed up. It's called passive advertising. Every comment you make is essentially an advertisement, and the more pleasant you are as a writer, the more reason they have to look at your ideas.
Are you a staple in the community? Most successful faction admins have good relations with the site staff. Why is this necessary? It makes it easier to talk. Introduce yourself. Be pleasant. Tell us when you have a problem with something, and we're more open to work with you. This community operates day to day on the backs of the Staff members who volunteer hours of their life for the purposes of entertainment for the community. Treat them with respect and dignity, and the same will be shared with you when you wish to discuss your ideas. Otherwise, you'll slowly turn us into DMV employees whenever you come knocking asking for something.
And lastly, advertising to your friends outside the community? Obviously, this is the big one, as we don't ask anyone to advertise SWRP - we handle that on a much larger scale for all of us. However, the most assured way to get your idea going is to spread the word yourself, and spreading it to newblood is the easiest way to go about this. Everyone loves hanging out with their friends, and there's no better way to spend time building a faction than with your own friends and confidants. I know this is the hardest of the advice given in this post, but it's the most assured way to gain support. This community is about building friendships and role-playing, and this is the core of building a faction.
With that said, something Staff could do is begin to do a "Faction of the Month" category on the sidebar visible to everyone who joins, sporting a cool image made by yours truly to help advertise smaller factions.