Every parent wants their child to love the new baby. And while there are many little things you can do to smooth the transition (giving big brother/sister gifts, not holding the baby when they first meet, etc.), the key to it all is ATTENTION. Regardless of the age, kids need one-on-one attention and reassurance that they are still important.
Many kids, no matter how gently the initial introduction is made, feel threatened by a new little person who is suddenly in the spotlight. To better empathize: "Imagine your spouse announces that being married has been such a great experience, he has decided that it would be even better to bring in an additional wife." (Dads, just reverse it and imagine an additional husband on the scene.) Talk about a game-changer! Oh, and you won't mind sharing your things/toys/room, right?
The second most important thing to keep in mind when preparing your child/other children for a new baby is maintaining a consistent routine. Having those familiar touchstones in place, helps orient them during the upheaval. This bears some thought before the baby's arrival: i.e. if possible, get dad to take over the bedtime routine so it won't be as disruptive when mom can't.
If they are not in school or child care program, consider having dedicated care for the first month or so. Grandparents, babysitter--just someone who can give them undivided attention while you recover and care for your newborn. (Of course, this is always something a Postpartum Doula can help with!)
From my unique vantage point, I can attest that regardless of age and temperament, the siblings who transition best, were the ones given the most attention and one-on-one time.
For further reading, "Siblings Without Rivalry" is a tremendous resource. If you've already been down this road, comment below & share what helped your family most.
Postpartum Doula, Breastfeeding Counselor, Mother of Three.