Resources for practicing Fermi problems?

Hello,

I had a really important interview today and managed to completely embarrass myself on a Fermi problem. I've never been really good at them, but this one... worst answer the interviewer ever heard. I did better on the second Fermi problem-calmed down and tried to go slower-and got a decent answer, but I still didn't do as well as I hoped. I needed a little nudge from the interviewer.

I actually realized that-once you took away the massive level of nervousness I was undergoing at the time-that I found the Fermi problem kind of fun. More importantly, I don't ever want to screw up like that again. Part of this is simply improving my verbal communication skills, which have never been good, but part of this is getting better at these sorts of teasers, which is a nice way of testing my logic skills, too. So: does anybody know a resource out there so I can get better at this? This might actually be a nice relaxation from my other studies.