As RomeoStevens said, practice is your best bet, but even better is to practice with people who have a decent grasp of what prospective employers and HR people will be thinking while interviewing you.
There are surprisingly many community centers that offer direct training programs for finding good jobs and winning interviews, and they almost always include at least a little bit of interview practice. Up here in Canada, they're almost always free thanks to government funding, but for U.S. it'll depend on the local specifics, I'd wager. However, you want to be looking at the very local stuff, i.e. town/district or community newspapers, the city hall's list of community organizations, the local section/version of the "yellow pages", and so on. These are the places where you're most likely to find or learn about the kind of places / community centers that offer these training programs.
But again, the key point is practice and experiments with good feedback (most honest feedback is good feedback, but much more so if the other person(s) are experts in this, which is much more likely in the aforementioned community centers).
Live practice, as RomeoStevens mentioned, was definitely on my list of things to do to improve interviewing skills. Your advice about seeking out direct training programs was something I did not even consider, so thank you! That would indeed seem very helpful.
With Alicorn's permission, I'm resurrecting this thread.
I'll start off with one of my own: What kinds of exercise can I do at home (I do have 5- and 20-pound weights), and what are good ways to get motivation to do so regularly?