Ask HN: How do I prep for interviews, and stay calm before?

I know there's advice all over the Internet for this, but still just hoping for some feedback. I just finished a Skype interview that I'm feeling horrible about and this isn't the first time - I constantly feel terrible when doing coding interviews, I was even hesitant in my basic Fibonacci implementation (which was just a warmup!)

I can't let this keep holding me back, so any words of wisdom on this are appreciated.