We're in a similar boat.. but remember it's not the 50's and 60's where parent felt it was a required responsibility to do scouts, youth group, PTA, social clubs. In the this current world.. parents need to be reeducated that "volunteerism" is not always "time needed" but a general responsibility, whether it be a "Hi, how's the (insert project here) going? , telling family, friends, neighbors about school functions, happenings. It can also require time though. Like taking a phone list ( and calling people on it), assisting in the classroom, being on a committee... Unfortunately (or fortunately for some) a lot of the schools out here require parents to do XX hrs of volunteerism (PTA or classroom).
If your school has a parent room/center (We are finishing ours) It seems to help- a base of operations, goings on per se..
And remember "Could you help for an hour..." and "Thanks for your help.." (I'm a naturally talkative person so everyone at school seems to find me to ask about PTA/ school stuff) Be visible and friendly to all (carrying a big stick also works fro me on occassion)
Lastly, use the Dad, Father, Male resource- we've recently done a few Donuts for Dads (free donuts and coffee then hook' em in) and the big thing I get is "we want to help, but dont know who, what, where, when or what TOOLS you need.. (i've also seen put out flyers directly addressed to dads or males- it works better, but as a 38yr old father of 5, I reallly don't mind if it's addressed to mom, dad or family..
Enough from me for now. Good luck hope something I wrote helps