Some wonderful advice here.
One "trick" that has always worked for me is simply to acknowledge it....not to say "I suck at interviews" but "I have to tell you, interviews make me a little nervous so if I start to ramble please reel me back in", smile, and then add a cherry on top.....like.....I've been reading about your new project x and am really interested in hearing more about that, or something else that puts the ball back in their court for a minute.
Most people get nervous in interviews....and HR folks know that better than anyone....acknowledge it, take a breath, and move on. Somehow it just makes it easier....especially if that person interviewing you says "oh my gosh, me too, don't worry about it, just tell me..."