From Quote Geek:

• You must be the change you wish to see in the world.
  Mahatma Gandhi
• Never doubt that a small group of thoughtful committed citizens can change the world: Indeed it’s the only thing that ever has.
  Margaret Mead
• Volunteering is for suckers. Did you know that so called ‘Volunteers’ don’t even get paid?
  Homer Simpson

From Wikipedia:

• Words without actions are the assassins of idealism.
  Herbert Hoover

And from a source I didn’t note:

Please feel free to post your own favorite volunteer quotes as comments.