ModelAdmin.save_model(self, request, obj, form, change)
The save_model method is given the HttpRequest, a model instance, a ModelForm instance and a boolean value based on whether it is adding or changing the object. Here you can do any pre- or post-save operations.
For example to attach request.user to the object prior to saving:
def save_model(self, request, obj, form, change):
obj.user = request.user