Another one in the category of always-forget-how-to-do-this-so-noting-here:
class Entry(models.Model):
blog = models.ForeignKey(Blog)
b = Blog.objects.get(id=1)
b.entry_set.all() # Returns all Entry objects related to Blog.
# b.entry_set is a Manager that returns QuerySets.
b.entry_set.filter(headline__contains='Lennon')
b.entry_set.count()
Full docs here.