R.D.
Prepare them. Let them know what's going to happen when baby comes. Let them know you'll be tired and busy, but you'll still love them just the same. Get them excited to be your little helpers. Give them specific, age appropriate things they can do to help (bring diapers, grab a bottle, whatever). If they're helping, they'll feel a sense of pride and accomplishment, and they'll also feel a sense of 'ownership' over their new infant sibling :)