Our school has a bake sale run by the grade 6's every month. They're fundraising for the big grade 7 trip they'll go on the following year. The teacher sent home at the beginning of the year a list of dates, with 5-6 kids attached to each date. The letter said - We're fundraising for a trip. This will save you money next year. Each child is expected to contribute baked goods on their assigned date. That worked. They did it, even if it was just giving them cash for Tim's/a dozen donuts, they did it.
People are only going to contribute to causes that they care about. People will only go out of their way for causes they REALLY care about. I can't think of anything on earth that would motivate me to contribute to a bus driver appreciation breakfast. Some people will flock to help with the Halloween party. Some will come in droves to help with Christmas stuff. You're gonna find that people will flock to help with some things, and noone will help with other things. So you plan on doing some things with no help, or just don't do them, especially if there's no support from them within the community.
A sneaky but effective way is to really hype it up with the kids. So they go home and beg and plead with mom and dad to go help. But I find that one cruel to the kids - the ones whose parents can't or won't for whyever are really sad and embarrassed. It will get some parents in there though, however temporarily.
Also, our school puts on a thank the volunteers breakfast every year. The principal hosts, and all of the teachers are there. There is a lovely little invitation which looks as if you're being invited to High Tea, and a nice breakfast, with a thank you speech from the principal. While it isn't much of an incentive to do it the first time, it certainly did give me incentive to go volunteer a second year, knowing that my effort was actually appreciated. Some extra attention at the end of the year will do wonders with turnout next year.
Otherwise, if they're not already volunteering, they're prolly not gonna. If they see that the room mother is nice, and they're not gonna be harassed forevermore if they just want to try it once, they just may give it a shot. But for most, I expect you're SOL, and as the kids get older, it'll get worse.