Ask HN: Best practices for securing clustered services

I'm currently setting up a scalable application and was wondering what the best security practices are for a multi-server setup.

Currently I use SSH key authentication on each server - but how do I handle passwords for sudo? Should I disable sudo password checks? Should I use the same password for every server and rotate it every week? How about database passwords? Should I remove the need for a password by SSH tunnelling to the master DB server?

Thanks in advance!