The Humane Society of the United States has a few suggestions to prepare your pet. Some of them sound a little silly, but they do work.
One of the most important things is getting the pet used to the new noises and sounds the baby will make - a lot of pets will find baby cries and the like upsetting. You can play crying sounds and the like to acclimate the pet, for example.
As for actual introduction of the baby to the pet, they suggest first giving the animal something with the baby's smell on it (a blanket, etc) so they don't get alarmed by the new smell in the house, if possible.
It is also suggested that you try to keep your pet's routine the same as much as possible, and that you greet the pet first without the baby when you arrive home with your new child.
Basically, you want the pet to interact positively with the baby, so you will want to reward and encourage positive interactions, and at first you might want to limit the time the pet and the baby spend actively together. Bring the baby to sit with the pet, etc (with you holding the baby, of course), for small bursts, extending them over time, as the pet gets used to the baby.