Getting people active on your forums, and keeping them that way, is a tricky problem, and one that even we and other large alliances aren't immune to.
Part of the problem is inherent in PT - it's a young game, with relatively few engaged (internationally) alliances. There isn't much drama / stuff happening, so people aren't as interested, and don't always visit the forums.
Below are a few suggestions to help remedy the problem.
Populate your forums.
Try setting up a lively spam section, debate section, gaming section, whatever. A place where people can come to chill, talk about stuff, and get to know each other. This gives them things to do (and reasons to show up at your forum) when PT is slow, and builds a stronger community.
It won't be easy at first, but try and keep the threads you throw up as active as possible - even if its just you and the 4 other active members posting in it, when people come to the forums and see that there is activity, they will be encouraged to be active. On the other hand, if they show up and see the last post in the discussions area was 3 months ago, they probably won't come check it again or get involved.
This probably isn't that tenable at your size, but when you are larger, you can divide the alliance into different groups and hold competitions. This gives your members something to do within the community, but outside the game - ensuring traffic even when the game is slow.
We regularly hold "House Wars" - SK members who want to participate are divided into the great houses of Westeros (currently only two at the moment), and every week face off in competitions (winners get a medal for their signatures, bragging rights, and occasionally in-game rewards) ranging from competitions to raise their daily income or growth, to games of Afterwind, Risk and Quake Live, to recruiting and design contests.
Encourage Activity Outside the Game.
Make posts asking people what game they play and want consoles they have. If a lot of people have xbox's and like halo, organize some times where you all get together and play. If everyone loves online Risk or Afterwind (great game, btw, I'd check it out - a lot of SK people play together), League of Legends, etc organize times where you all play those together.
This will strengthen your community, which will make your alliance stronger and increase activity across the board (IRC, forums, etc).
The most active communities are the ones that are closet to each other. I'm more likely to go to the forums and hang out with people I know and like than a bunch of strangers on the internet. While you can get to know one other over forums, the best way to do so is over IRC, where people can really talk to each other and hang out.
Once your members get on IRC, they may also start heading to the main PT channel and the channels of other alliances, and generally getting more involved in the PT community and PT politics, which will make them more interested in the game, which will drive up activity, etc etc.
Enforce Activity Requirements
If you find people aren't coming to the forums and IRC, you can start requiring that they do if they want to stay members. Tell them everybody is expected to post on the forums at least once a week (you can raise or lower that), and highly encourage them to use IRC. If you try this, make sure to start on my other suggestions first. While this will drive people to your forums, if there is nothing there for them to do, it wont keep them there, and wont boost activity.
Those are our main suggestions for activity problems. They may work wonders, they may not. Activity is a very tricky thing, and something every alliance grapples with. It depends on the members you currently have and how much they are willing to really be active, but trying these out can't hurt. If nothing , these are good ways to energize the active members you do have.