When our second was born, we brought a gift bag to the hospital filled with our 3yo's favorite things-(all individually wrapped)playdough, crayons, puzzles, gum, etc... After the baby was born we told him the presents were from his little brother. He was SO excited, because he really did think they were from him (still does!). It was neat watching him unwrap all of the presents and getting so excited that the baby knew what he liked!
When the baby was born, my SIL bought both the boys matching little shirts that said, "I'm the big brother and his name and a little cartoon picture of a boy on it. And the baby body suit said, "I'm the little brother and his name, with a cartoon picture of a baby on it. They are the cutest little shirts ever and our oldest loves wearing his, especially when baby has his on too!
I think the present idea really helped a lot because he has not shown one ounce of jealousy towards the baby and it's been 8 months. He really loves his baby brother. We also have instilled in him, that part of his "job" as a big brother is to watch over baby, and to help protect him. Also, to always show him love and kindness. It's so sweet watching the two of them together because there really is a brotherly bond happening with them already!
Good luck to you and your babies, it's fun watching them interact together! :)