Taking a positive approach
Try not to punish your older child, as this may lead to them feeling more resentful towards the baby. Although you must explain clearly that they are not allowed to hurt the baby, tell them you know they are not meaning to be naughty and should tell you how they are feeling, rather than taking it out on their brother or sister. Hopefully this should encourage them to open up to you a bit more and feel comforted by the fact that you want to know how to help them feel better.
By responding with patience and understanding, your child will start to feel secure again, and know that by making room for a new member of the family, they are not giving up their place or getting any less love from you. This will help them to accept the baby and start to see that they are also a part of their life, in their role as big brother or sister.
If friends and family are buying gifts for the new baby, it may be an idea to get some gifts to give to your child too so they don't feel left out. You might also want to make up a little box full of treats and fun activities, books and toys from the baby to your child.