Evangel, the evangelical blog at First Things has a nice series of posts entitled Augustine on Christians and Civil Government. Enjoy these goodies:
1. Introduction: Christianity and Politics
2. Augustine on Civil Government: The Two Cities
3. Augustine on Civil Government: Two Further Preliminaries
4. Augustine on Civil Government: Authority
5. Augustine on Civil Government: The City of God and Compromise
6. Christian Political Participation
7. Religious Motivations in Politics