PTO Mom and Dad I think touched on part of the problem. People want to be part of a group that is having fun and they definately want to feel welcomed. The first thing I tell the new Presidents (and the same that was told to me when I took on Council President) is FIND YOUR REPLACEMENT! That means, inviting them to the meetings beginning at the year. Have them see what you do in your position as you do it. Show them the rewards you receive in doing it. (Emotionally etc) and rarely complain about it. If they hear you saying, "It's so much work and we're getting burnt out and the parents don't care" would you want to sign up for something like that?
I also see the largely growing Latino community in some of our schools and that is a big hurdle. Their time may be very limited (many working several jobs just to pay the bills) or the language barrier, or culture differences (school parental involvement isn't big in some of the cultures and PTA doesn't mean anything to them)
You need to assess what your obstacles are like PTO M&D said and work on it from there. The short term fix could be the letter that laurib did but you need to check the problems underneath eventually.
Speaking of assessments, I posted a topic about an assessment that National PTA provides. You do not have to be a PTA school to do it and it might help you figure out where you need that help. Pals has done it (at least that was last I heard) and would be great at explaining how it worked for them.
I couldn't find the topic but some info is at
www.pta.org/parentinvolvement/certification/index.asp
You will need the cooperation of your Principal to complete the assessment.