I'm ASA certified, but have been asked to pick up a handful of scrimmage games over the next few weeks for Middle School using NFHS. Is there a cheatsheet somewhere that explains the main differences?

Don't know of any "cheat sheet" to give you the differences, but here's two I can think of right now. First, the pitching rules. In NFHS, the pitcher can step back off the rubber with her non pivot foot. In ASA both feet have to remain in contact with the pitching rubber. I'm probably not wording this very well. I'll try to find my High School rule book and get you the exact wording. The second one is on interference by a runner on a foul fly ball. In ASA only the runner is declared out. In NFHS I think both the runner and the batter would be out. I'll have to double check that one, too.

One more I just picked up off eteamz. NFHS does not allow team huddles on the field between innings. So if coach wants to give a pep talk to teams coming off defense or going out to defense, has to be done in dugout.

Handbook Rules Differences
 

The Umpire Handbook of Softball Rules Differences [For 2005 v. 6].

It discusses ASA [FP], NFHS, and NCAA rules differences. I find that it is a handy reference.

Try here:

http://www.dcasa.org/images/2005NFHS...ifferences.doc