http://chriskief.com/2012/12/17/subclass-a-django-model-form-and-add-extra-fields/

forms.py

from django import forms
from app.models import User
from app.forms.widgets import *


class RegisterBaseForm(forms.ModelForm):

    username = forms.CharField(max_length=User._meta.get_field('username').max_length)
    email = forms.CharField(max_length=User._meta.get_field('email').max_length, widget=HTML5EmailInput())
    password = forms.CharField(min_length=6, max_length=16, widget=forms.PasswordInput())

    class Meta:
        model = User
        fields = ('username', 'email', 'password')