Congratulations!!
As the other responder said, it wasn't really a big ordeal when we were preparing for the birth of our other two children. With my oldest, he was turning two when his brother was born. We told him about the baby, let him talk to and feel my belly, and he got a "big brother" shirt at the hospital and "big brother" present from grandma and grandpa. He was so loving to his baby brother right from the start - almost too much so, we would really have to watch him because he would try to pick him up and give him really big cuddles that would squish him!! He even would try to nurse his monkey lovey while I was nursing the baby!!! This would make me laugh and it would make my husband cringe! :) My husband finally said we should just give him a bottle he can use to feed his monkey and let him know that's how daddy's feed babies! It was really cute!
We pretty much did the same for our boys when our third boy was born (they were 5 1/2 and 3 1/2). We told them how important they are as big brothers, we brought them to one of the ultrasounds, they would talk to my belly, and we read a book about being a big brother. Right off the bat, they were great with the baby!! Again, so much love there - we've not had any problems with jealousy at all.
I do agree that sometimes people can go overboard with preparation and can cause more problems than if a big deal wasn't really made about it all. Definitely, the kids should be included in the process of preparing for the baby and helping take care of the baby (ie. reading books, helping decorate the baby room, maybe giving them a small gift after the baby is born, letting them help you by getting a diaper, blanket, or bottle for the baby). I think this makes the older siblings feel important too!
Good luck with everything!!