But I wanted to point out that the opinion expressed in the accepted answer is somewhat outdated. According to more recent discussions (django bugs #7634 and #12785), auto_now and auto_now_add are not going anywhere, and even if you go to the original discussion, you'll find strong arguments against the RY (as in DRY) in custom save methods.
A better solution has been offered (custom field types), but didn't gain enough momentum to make it into django. You can write your own in three lines (it's Jacob Kaplan-Moss' suggestion).
from django.db import models
from django.utils import timezone
class AutoDateTimeField(models.DateTimeField):
def pre_save(self, model_instance, add):
return timezone.now()
#usage
created_at = models.DateField(default=timezone.now)
updated_at = models.AutoDateTimeField(default=timezone.now)