Our PTA had woefully small numbers attending the meetings-the executive board of about 8 including the principal and VP, and even they wouldn't all show up sometimes-- for a looong time. It started out as parent apathy, but later turned into the "clique" issue.
I agree with MichelleB that part of the problem is everyones' too hectic lifestyles. I admittedly didn't go for years though because I was too intimidated by the "clique." It's unnerving even for the most well-adjusted person to walk in a room full of people you don't know, and then be ignored by them--even if you bring a friend. Then a friend of mine got involved with some of the activities, she got me involved, then it sort of turned into the old Clairol commercial ("and then she told..."! Not that we have a huge turn-out every meeting, but it IS getting better (about 20-25 on a regular basis).
I'm finding from our PTA, and from the volunteer group I've organized in our school, that participation relies on people actually caring about what happens in the school and feeling that what they have to say matters. Too often I've heard people say, "Why should I bother going? "They" have their minds already made up, and nothing I say will change anything anyhow." I'd like to see more education of the masses concerning the benefits and fun of being "involved" AND also gently reminding people that they really SHOULD be involved. It's just too easy to be lazy, and I speak from experience!
That really doesn't answer your question though, does it?! I'd say you could try being more vocal, literally!
Talk to people, make sure they know the meeting can be more than just R's Rules. I bring coffee and donuts, and while that doesn't lure people, it does make those there happy. I also good-naturedly harass people I know, and encourage those I don't know, to come--I try to emphasize that it's a very friendly group of people, and that they shouldn't feel intimidated. The PTO TOday article about groups being cliques makes a good point--even if you're not a clique, you have to ACTIVELY strive to make sure people know you're not. That article also had good tips about running meetings. I also tell everyone involved to make a friend come with them to the next meeting. That old Clairol commercial does make a point!