When did your baby start sleeping in a crib?

My son is 4 months old.. and I am thinking that he is probably growing out of his binet.. or will soon. What age did your child begin sleeping in a crib and what method did you use to make it go smoothly. Currently, he naps fine in his crib. I place him in it with lullabies and he can usually put himself to sleep. Suggestions please.. thanks.