Django ModelForm custom fields in ModelAdmin

gistfile1.py

from django.contrib import admin
from django import forms

class MyModelAdmin(admin.ModelAdmin):
    # ...
    def get_form(self, request, obj=None, **kwargs):
        form_factory = super(MyModelAdmin, self).get_form(request, obj, **kwargs)
        form_factory.base_fields['my_custom_field'] = forms.CharField(widget=forms.Textarea(), required=True)
    # ...
    def save_model(self, request, obj, form, change):
        obj.save()
        # do your custom processing on 'my_custom_field' - maybe save it to an alternate model,
        # or check it for validity.