In the admin interface and newforms there is the brilliant helper of being able to define choices. You can use code like this:
APPROVAL_CHOICES = (
(\'ye
@JonnyBuchanan gave the right answer.
But if you need this in the django admin for many models, and you're (like me) too lazy to customize a ModelForm and ovverride the right methods inside the ModelAdmin class, you can use this approach:
http://www.abidibo.net/blog/2013/04/10/convert-select-multiple-widget-checkboxes-django-admin-form/