I've gone through the getting started tut for python27 and app engine: https://developers.google.com/appengine/docs/python/gettingstartedpython27/

By the end of the tut, all the the classes are in the same file (helloworld.py) and you and you configure the router to point a url path to a class at the bottom of the file:

 app = webapp2.WSGIApplication([('/', MainPage),('/sign', Guestbook)],debug=True)

What the tut did not cover is how do I orginise my classes / files as my app grows. For example, would I put MainPage in a separate file and then call 'import MainPage' in the helloworld.py file and add the route to the WSGIApplication? Is there anything more automated than this? What should I call the MainPage file and where should I store it?

Preferable to importing all of your handlers at app-startup is to take advantage of webapp2's lazy handler loading which loads modules/packages as needed.
So you have a couple of options:

Option 1, Handlers in a module
Place MainPage in another file (module) at the same level as your helloworld.py file:

/my_gae_appapp.yamlhelloworld.pyhandlers.py

And in your routing (in helloworld.py) you would do:

app = webapp2.WSGIApplication([('/', 'handlers.MainPage'),('/sign', 'handlers.Guestbook')],debug=True)

Option 2, Handlers in a package; perhaps consider as your app gets larger
As your app gets larger you may wish to create a package in which to place your handlers:

/my_gae_app/handlers__init__.pyguestbook.pymain.pyapp.yamlhelloworld.py

Routes (in helloworld.py):

app = webapp2.WSGIApplication([('/', 'handlers.main.MainPage'),('/sign', 'handlers.guestbook.Guestbook')],debug=True)