Partly it was just experience. Interviews (as done poorly in the tech industry) tend to follow certain patterns, the more you do the more you see the patterns and the more you can adjust for them.

Perhaps most importantly (and this is something that is often mentioned by people give interview advice but easy to ignore if you're a "meritocracy"-minded techie) I've gotten really good at just sort of taking control of the interview and leading it where I want it to go (while still being sure to display how I would be valuable to the company) rather than being a passive question-answer-er, but getting good at this is also pretty much just down to experience.