It may seem old-fashioned, but I still believe that good people can do great things from elected office. Put together a platform you believe in and go for it. Volunteer on a campaign. If you don't want to run for office, volunteer for someone who does. It can be in a local campaign or in the local branch of a national campaign.
Campaigns are always short on money, and volunteers are the heart and soul of most efforts.
Talk to your children about public affairs and politics. Make sure your dinner-table talk goes beyond Power Rangers and the prom. Civic discussions are too important to be left to television talking heads. Write letters to the editor. Give newspaper readers a chance to read something more intelligent and insightful than the normal editorial goop.