Get Yourself in School Shape
Come and go. For a few weeks, leave your child with grandparents or a babysitter more often than usual, and show him that you'll come back when you say you will.
Be on time. If you get to school late in the morning, it'll make your child feel anxious. It's equally important for you to be five minutes early for pickup (but don't let your child see you before dismissal). It's very hard for a child to be the last one left after everyone else has gone home.
Have patience. Some children will get used to school after a day, while others take several weeks to feel at home. Keep in mind, though, that if your child starts crying when he sees you at pickup time, it probably means that he's just happy to see you -- not that he had a terrible day.