Nothing can completely prepare your baby for a new brother or sister. The arrival of another little person is a huge challenge, especially for young toddlers. And no matter what you do, your firstborn could get a tad jealous. But there are ways to help ease the transition:
Shh! Be quiet. There's no need to tell your toddler you're having a baby the second you get your pregnancy test results. Given his inability to judge time, he'll end up spending the next several months waiting for his world to change at any minute.
Plan ahead. Minimize the displacement that your first child might feel by getting new plans and relationships underway before the baby arrives. For example, if you've always been the one to tuck him in at night, start having Dad or Grandma help out with his bedtime routine. Any other big changes -- such as moving to another bed or bedroom, getting a babysitter -- should also be done now rather than later.
Keep it short and sweet. As your due date approaches, you can mention the new baby more frequently. Speak in simple terms and always emphasize the place of your firstborn ("A new baby is coming. This will be your bed and this will be the baby's bed"). Since toddlers like stories, you can also start reading him books about babies and new siblings. Sometimes children think that new babies are like new cars, and that you have to sell the old one to get a new one.
Say the magic words. He may still get upset, but always remind him how much you love him.