If your toddler is very young still, the best idea is to get him or her used to the idea gradually. Buy books that have babies in them, or take him or her to visit a friend or relative who has a baby. Explain that your expanding tummy is where the new baby lives for now, but that soon; there will be a new brother or sister to play with. You can even show your toddler pictures of the new baby taken during your ultrasound.
As much as you are trying to prepare your child, however, you need to understand that he or she is still likely to be a little resentful of the new arrival.
Make sure that you are still spending a lot of time with your toddler, and that he or she still feels special. Include your toddler in caring for the new arrival, by letting him hold the clean nappy during nappy changes, or by reading books to him while you are breast-feeding the new baby.