class FlatPage(Model)
    ...
    sites = ManyToManyField(Site)

sites = ManyToManyField(Site, default=Site.objects.get_current)

class ExtendedFlatPageAdmin(FlatPageAdmin):
    fieldsets = (
        (None, {
            'fields': ('url', 'title', 'content')
        }),
        ('Advanced options', {
            'classes': ('collapse',),
            'fields': ('enable_comments', 'registration_required', 'template_name')
        }),
    )

    def save_model(self, request, obj, form, change):
        obj.save()
        current_site = Site.objects.get_current()
        obj.sites.add(current_site)

$('#id_sites').attr('selectedIndex',0);