Ask HN: What is the best way to deploy changes to code?

Have you had good experiences with any particular tools for code deployment? What principles should I adhere to when deploying changes? Is there a good way to combine unit testing with deployment?

My hope is to devise a mechanism to ensure that what is in our code-base stays in sync with what is in the working environment with minimal complexity and reduced change associated risks. I don't want to be too specific or else this would become more of a stackoverflow or google question - if it hasn't already.