I usually write up the campaign premise/intro as if writing the leader page to a story. I then follow this with the gnarly bits, like acceptable source books, character creation methods, etc.
The first part is to set the flavor, get people into the mind set of that world/campaign. The rest is the backstage activity needed to support that facade.
I posted the campaign intro for our current game over in the Story Hour forum, along with a number of our adventures in story form.