How do you structure i18n yaml files in Rails?

I started populating an en yaml file in Rails and I can already tell it will get messy and out of hand before too long. Is there a convention to keeping this file organized?

So far I have this structure:

language:
  resource:
    pages: # index, show, new, edit
      page html elements: # h1, title
  activerecord:
    attributes:
      model:
        property:

Now I have the following things that I want to fit into this structure but I'm unsure how to:

1. Navigation
2. Button text (save changes, create account, etc)
3. Error messages from controller flash
4. How to add multi-word keys. Do I use a space or an underscore? For exmaple, t(".update button")) or t(".update_button")

Is there a convention to locale file structure?