Some tips from someone who's been a part of a few:

1) don't bother with behavior rules. the general public, even kids, tend to be quite respectful and fascinated by the machines, and treat them pretty delicately. I've had no problems with type-ins at restaurants and coffee shops. The inevitable "carriage return into a coffee cup" will happen, but it's the typed sheets laying on the table that get damaged. We know how to clean our machines. (:

2) as beautiful and wonderful as Hermes and Olivetti machines are, some models will have problems in a public environment. Anything with a Magic Margin-style setting will frustrate newbies (H3000, many Royals, etc.) and Rockets and Letteras have linespace levers and carriage lock systems that are easily set to a position that will lock the carriage up and frustrate people. You learn pretty quickly which ones you have to keep an eye on to make sure you don't get the guilty "I broke your antique typewriter" look of despair from some innocent budding typist. (:

Smith-Coronas, Brothers and Olympias always seem to be a safe and sturdy option for newbies to play with, and I recommend them for the main "public typers". Older iron machines, if working well, are always popular and tend to have simple controls that are hard to screw up, as well.

I did a Type-In in Manchester, NH a few years ago. It was a quirky and new thing, so I gave the local newspaper a call and got them to do an advance. It helped get the word out.