#1
    def get_form(self, request, obj=None, **kwargs):
        Form = super(PostAdmin, self).get_form(request, obj, **kwargs)

        def init(self, *args, **kwargs):
            Form.__init__(*args, **kwargs)
            self.initial.setdefault('author', request.user)

        return type('Form', (Form,), {'__init__': init})

#2
    def get_form(self, request, obj=None, **kwargs):
        Form = super(PostAdmin, self).get_form(request, obj, **kwargs)

        class Wrapper(Form):
            __metaclass__ = type
            def __init__(self, *args, **kwargs):
                super(Wrapper, self).__init__(*args, **kwargs)
                self.initial.setdefault('author', request.user)

        return Wrapper

#3
    def get_form(self, request, obj=None, **kwargs):
        if obj is None:
            request.GET._mutable = True
            request.GET['author'] = request.user
            request.GET._mutable = False
        elif obj.author is None:
            obj.author = request.user

        return super(PostAdmin, self).get_form(request, obj, **kwargs)