Django Overriding Model Clean() vs Save()
You should use clean to do validation-related work, and to parse/change/otherwise clean the input.  Capitalizing fields and generating a slug can happen here.  I also use clean to force a field like post_type to a specific value in proxy models.  If you raise django.core.exceptions.ValidationError('error text') inside clean, the 'error text' is added to the  form.non_field_errors.  
Save is the place to change the way a model is actually saved.  For instance, I've used save to create a crop of an uploaded picture.  ValidationErrors are not caught if raised here, and I feel like that's the most important practical difference between the two.
I have a couple of actions to perform when saving a models, especially from the admin. I capitalize a couple of fields and check to make sure that either one field or the other is filled. I also create the field slug. RIght now these are split between overriding the clean and the save functions. It works now, but I am curious on when to use each. I looked through the docs, and I couldn't find specifically which to use when.