How do I set this up on Facebook?

The easiest way is to make a group of NotFarmvilleFriends and use that group when you post messages you don’t want your farmville friends to see. When you make posts using the Farmville app, click on the little “lock” icon, and choose that group to be the ones to see it. And, when you make normal posts, do the same for NotFarmvilleFriends.

Alternatively, you could also filter normal posts to be visible to “everyone except” Farmville Friends.

Posts that are already made, sadly, cannot be refiltered.

You could go through “privacy” – to do that, though, you have to state that only certain people can post to your wall, which means that your Farmville Friends won’t be able to see your wall at all – which, I’m guessing, would defeat the purpose.