I'm no rockstar, but I can honestly claim to be pretty comfortable in technical interviews. I chalk that comfort up to two main things.
1) A mentality that says something like...
"I'm good, I'm solid. Fuck this guy."
The point isn't the specific words, or expressing this outwardly, it's about getting into a mental state of calm confidence, pushing out the anxiety and concerning yourself only with the moment.
2) Talking with and being challenged by smart people on a regular basis. Ideally people who are smarter and/or more knowledgeable than yourself. I expect being the big fish in a small pond generally works against this.