Well, I've always mulled this around in my head (after switching from Planetside to WoW because of Planetside's rather dead player base) and finally decided to post it here, because I still consider Planetside to be by far a better game than WoW (interactive player versus player games are so much better than grinding three characters to 70 >_> )
Think about games like Silk road online. Those games are free. LOTS of people play games simply because they're free! I think if sony could cut it's losses for about six months and make planetside free, that the player base for the game would BOOM. I mean, once silk road got around by word of mouth, the game's servers were completely packed within only a few weeks! There is talk about Silk road going pay to play, and once they get so many players hooked on the game, they'll be able to draw them into paying for it.
Also, if the decision to go free for a period is not appealing, why not cut the price per month in half? If you get people willing to pay about 6 bucks a month (which is peanuts compared to 12-15 when people look at it) and you get twice your player base, You're making no loss. And when that happens, those people should spread the game by word of mouth even further!
Planetside is an amazing game. Just amazing. It's replability is endless, and I never grew tired of it. I just wish more people would come back to it. These methods would CERTAINLY bring the players back, I'm just not sure if Sony is willing to take the financial blow it would cost to keep supporting a game with no income (outside of advertising) for a few months.
Just my .02.