The first step in helping a child manage troubling feelings is to put yourself in the child's place. In his all-time classic child-rearing manual, Baby and Child Care, Dr. Spock demonstrated the merits of such an exercise. Paraphrasing him, I, too, would recommend that you imagine being in the following situation: Your husband comes home one day and says to you, "I love you so much and love being married, so I have decided to bring home a second wife. I will love you both equally. I have enough love to go around. It is because you have been such a joy that I want to do this again."
The momentary feelings of jealousy and helplessness that come over most wives while reading this vignette is what most older children feel when a new baby arrives. That doesn't mean parents should feel guilty for bringing a new baby home. Nevertheless, it is a shock. And it's not easy to share parents' love and attention. As long as we understand that, it should be easier to make the adjustment to the birth of a sibling less problematic.
When it seems appropriate, adults can point out how much better it is to be a big boy who can ride a bike than it is to be a baby who just eats, sleeps, and cries all day long. Teachers and parents should try to spend more time being play partners with the sibling of a new baby. Allow the child to take the lead, and don't be alarmed if he plays out some of his aggressive feelings. Accept what is expressed, and raise gentle questions about possible resolutions as you play.