I know a few things about'em. Right now I am Co-Gming [url=http://www.roleplayerguild.com/topics/19063-precipice-of-war-we-write-under-the-sign-of-the-goat/ic]Precipice of War[/url], which is one of the more successful NRP's in the section dedicated to them, and has been dragged more or less continuously across three different forums for more than four years now. That's my credentials. 

For a good nation RP, you have to break the tendency for people to think of the nations themselves as the characters. People do that and yeh, they'll RP like they are writing an AAR for Civ or EU. You gotta break them of that, and teach them to build their stories from the ground up by focusing on the individuals that make up the nation. Don't imagine cultures as CIA fact book stats, imagine them as individuals going through their lives. The government is the leaders and their personalities, the military is another culture of individuals with their own sets of challenges, etc etc.

It's best if you allow things to grow organically. Don't try to GM the RP using artificial tricks, just enforce some reasonable behavior. It's better to enforce realism within the world you are creating rather than creating barriers to power and access that might end up smothering the RP instead. The Big nations will be bogged down in logistics while the lesser ones are constrained by resources. Enforcing these sorts of things won't kill combat and action, but rather will keep it focused so that storytelling can blossom. 

And the best thing a GM can do, I think, is lead by example. Write well and you will inspire everybody else to keep up with your skillz.