Spughy - my sister and I are 6.5 years apart. - but my mom did have a still birth 2 years before that - I remember more of the prep for the first sibling, and I got a book, where do babies come from . . . I think alot depends on the temperment of your oldest child. If they want to help, let them help. Some people are very much insistent the older kid doesn't help, only play with the new baby, and I think that's only a good route if they don't want to help, or might come to resent it.I really wanted to help my mom with the new baby, but she wouldn't let me (I think it may have just been a bit of overprotectiveness given the previous still birth, which is understandable to me now, but I resented it as a child).
My son was just under 2 when my daughter was born, we talked about the new baby, he came to the ultrasounds, but I think he didn't really get it. He felt her move and kick in the womb, and I let him listen to my belly with the stethoscope. Most of these things just kind of freaked him out, though he still likes the stethoscope. This time, he will be nearly 3.5, and I plan on taking him to the sibling class at the hospital, getting a book, and perhaps a doll. My daughter will only be 17 months, she will definitely be getting a doll for Christmas, and well, I don't know there is much more that can be done for her that she'd even comprehend.
I don't think either child will be coming to the ultrasound this time. With the last baby, my son was entirely freaked out by me being on the table and cried until he could sit on it with me. This was not really a great thing for me or the tech. I don't want to risk that this time, so I think he will stay home, and he can watch the dvd later.
What helped most I think, was including him as much as I could once the new baby was born. He sat with me during nursing sessions, fed his own stuffed animals, helped with diaper changes when he wanted to.