Continuous deployment in 5 easy steps

Two options to deal with the "scary" part of big integrations in continuous deployment:

Don't. Commit early, often and in small chunks. Just search for Continuous Integration, more than enough has been said on it already.

When you must, roll out features independently of the code being live in production. This means things like using control logic to roll out the code to portions of users, intelligently. We roll out via: on/off switches (mitigate total failure), A/B tests (mitigate poor performance in front of customers), QA-rollout (mitigate bad experiences for customers), etc. Lots of flexibility here, including hidden rollouts: running the code without showing the results to users, so you can figure out scalability impacts.

Yes, I've had good luck with branches for trunk, qc, and production. The production branch is always a copy of qc as it existed at some point in the past, so that guarantees that everything's been tested together and without anything extra.

Beyond the benefits of just automating deployment (independent of source control and continuous integration), it's convenient to use source control as a historical record about the state of the production systems, and it's good not to have to remember rarely-used combinations of parameter values in deployment scripts.