I volunteer weekly with a local adult literacy centre and also volunteer for various charity events.
One suggestion I have is that volunteering is almost always fun with more people. Last December a few local improvisers and I volunteered for a turkey drive - encouraging folks at a grocery store to buy turkeys for a food bank. We had so much fun that we set up an informal group that signs up together for these sorts of events, volunteers as a group at places like food banks and have just started doing things like park cleanups. As a group we do the thing we set out to do but try to pair it up with something fun - the park cleanup, for example will have a campfire and snacks at the end (those who want to come to that but can't make the cleanup can bring a canned good for the food bank instead). Often, it seems, the people stopping people from doing volunteer work is simply inertia and not knowing what's out there. Bring the event opportunities to them and they'll come. Make it fun and even more will.