There really is no concrete way that they can affect policy decisions in Japan. This is changing slowly. Public disclosure laws are popping up on local levels around the country. There is debate over a National Freedom of Information law, although that was proposed back in the Hosokawa days and hasn't really gone anywhere since. Citizens groups, also, who are trying to come down hard on the practice of local bureaucrats wining and dining central government officials on public expense.
Japan has a long way to go.
I don't think that change is going to happen overnight. I never really saw the public clamoring for politicians to take charge in policy. This was a question I was really interested in, so every time there was an election, I would go to local areas and talk to voters, asking them what they wanted from their politicians. And I must say that almost every single one of them wanted their politicians to take care of them. In a Japanese context this meant attending their parents funerals, finding jobs for their children, or providing a pipeline back to the Kasumigaseki Ministries so that their business could get a small piece of the next Public Works project.