I could adopt several new handles and personalities, thus inflating the number of active members...
But any other way just ain't gonna happen, unless you include more games. Not only is FS OLD, it's now REALLY OLD, game time, and even if it were bought with any frequency (FS1 is still sold quite a bit, so I assume THAT is) people have no way to find us. Get Volition to put up a link on their page, or get one of those search engine registration programs and take the top of the list on Google and Yahoo for "FreeSpace". It won't be worth it, I can guarantee, unless we give out free FS2s to everyone who joins, but it's another way.
That, or make ME an admin and gimme my own forum.

Since I come free to all who wish to bask in my glory, it won't work that well for attracting people, but it might appease the E-Gods to have one of Their messengers in an admin capacity.