This is a shameless rip off (ie sincerest flattery) of Eric666's 3 page cheat sheet with the additional rules/units we use in our group, most of which were stolen from other player postings on BGG and Grognards.
A 3 page summary, condensing SPI's lawyerly presentation into something you can use as a refresher and quickly teach. The first page is an overview, the victory conditions, and setup, the rest is the heart of the game mechanics themselves, as well as some basics of strategy.