Create registration form for extended existing User model in Django 1.7

Question

I created extended User model in models.py as below:

class Employee(models.Model):
    user = models.OneToOneField(User)
    customfield1 = models.CharField(max_length=100)
    customfield2 = models.CharField(max_length=100)

and I want to add these fields to registration form. I analyzed this topic. This is the right solution but i can't apply into my site. I designed registration form but i can't register user. Codes i am using that is here: forms.py(This is my original form but this doesn't save Employee data into db):

class AdvancedRegistrationForm(UserCreationForm):
    customfield = forms.CharField(max_length=100,label="Address")
    class Meta:
        model = User
        fields = ('first_name', 'last_name', 'username', 'email', 'customfield1', 'customfield2')
    def save(self, commit=True):
        user = super(AdvancedRegistrationForm, self).save(commit=False)
        user.first_name = self.cleaned_data['first_name']
        user.last_name = self.cleaned_data['last_name']
        user.username = self.cleaned_data['username']
        user.email = self.cleaned_data['email']
        if commit:
            user.save()
        return user

views.py:

def register(request):
    if request.method == 'POST':
        form = AdvancedRegistrationForm(request.POST)
        if form.is_valid():
            form.save()
            return HttpResponseRedirect("/../login")
    else:
        form = AdvancedRegistrationForm()
    return render_to_response("register.html", {
        "form": form,
    }, RequestContext(request))

forms.py(This is the form that i create according to this solution but it doesn't register/save anything):

class AdvancedRegistrationForm(forms.ModelForm):
    def __init__(self, instance=None, *args, **kwargs):
        _fields = ('first_name', 'last_name', 'username', 'email', )
        _initial = forms.model_to_dict(instance.user, _fields) if instance is not None else {}
        super(AdvancedRegistrationForm, self).__init__(initial=_initial, instance=instance, *args, **kwargs)
        self.fields.update(forms.fields_for_model(User, _fields))
    class Meta:
        model = Employee
        exclude = ('user',)
    def save(self, *args, **kwargs):
        u = self.instance.user
        u.first_name = self.cleaned_data['first_name']
        u.last_name = self.cleaned_data['last_name']
        u.username = self.cleaned_data['username']
        u.email = self.cleaned_data['email']
        u.save()
        profile = super(AdvancedRegistrationForm, self).save(*args,**kwargs)
        profile.user = u
        profile.customfield = self.cleaned_data['customfield']
        return profile