Angie,
Do everything you can think of and things you cant think of!
Ok thats a little to broad, I know.
I have been faced with the same problem. A lack of involvement in PTO activities. A part of our solution was to work with the local community centers, we are lucky in that they have some great programs. Another factor was to make the PTO board more reflective of the racial make up of our student body!
I guess what I am getting at here is how can you expect people to get involved unless you go out and get involved with things that are important to them first? You may well be doing that, but as with my expierence in dealing with this same issue, I found that it takes time and a total committment to the community our kids come from to turn this around. It is not that these parents care less about their kids education either, its that they dont see the educational system caring about them much beyond are your kids at school on time.
As an example we support a reading program run out of a community center in a very poor neighborhood. This is something the PTO can do that the school may not be allowed to do (ours isnt because of limited funds). We also provide excess clothing (uniforms and non uniforms turned in to us) which is then distributed to those that have the greatest need. Talk to the community leaders and see what it is they need and what your PTO can provide. Shake a lot of hands, get involved in activities, politic. Ask parents directly to help out with events. Get the community leaders to ask them for you! Another example, when you all do the fall and spring pictures does the school or the PTO provide helpers that then get their kids pictures for free? Get those volunteers from the parents that you want to get involved that can least afford the pictures to begin with!
Ok done rambling, I hope this sparks some ideas and you can get the involvement that you seek.