TheyWorkForYou is a website that connects citizens with Members of Parliament in the UK. Following a scandal in 2009 when the government sought to have MPs’ expenses claims kept secret, MySociety mobilised voters to send a few thousand individualised emails to MPs demanding transparency in the use of public funds. Soon after this campaign, the UK government agreed to disclose data on MP expenses.

Open Source
MySociety in the UK created TheyWorkForYou website to connect citizens and MPs

This was a very large and successful campaign to access public information, but you might have something smaller in mind. Read about how to file an information request.