Talk, talk, talk. Go to the library and there will probably be 30 books FOR kids on being a big brother/sister. We got one book a week from the library. He went to all my doctor appointments (he left if I was having an exam he couldn't be there for, but if it was just a regular exam or especially if we were going to "see" the baby, he was always there).
I asked him to help me pick things out, to help me make choices on what we should do (should we put the bed here or here? this blanket or that blanket? this paci or that paci? etc) and he took a big brother class at the hospital. That was actually really sweet---they did a build a bear kind of thing so they could have a "baby" to take care of, they learned how to help mom, they learned how important and special being an older sibling is, got a cute certificate that we framed and hung on his wall, etc, etc. I got him a baby and we practiced things like how to hold the baby, where it is ok to hold a baby so he didn't get dropped, practiced rules and holding bottles, we changed stinky doll diapers and helped at bathtime.
Joseph was really good to go with it all....even after the baby was born and at the hospital.....until the baby came home. Fortunately, we had my mom and dad there. I had already prepared tons of meals and stored them in the deep freezer, the house was already cleaned by a housekeeper and I'd already washed all clothes and set up the nursery....there was nothing I really needed from my mom and dad (except an occasional load of laundry) so their one big "job" was to make Joseph feel special, play with him, he thought they came to visit him and everything else was just "extra". I also put him in a 4 hour, 2 day a week mother's day out program (which we called "school") as early as possible----which was about 3 months before the baby came. That way, it was already part of his "own" routine and not me sending him away because the baby came. He was VERY proud of going to school because we made a huge deal out of it. He got a new Spiderman backpack, lunchbox, and thermos because he was a big boy going to school.
(And when he went to my friend's house, who he adores, while I was in labor/delivery, we packed his special Spiderman backpack with some very special new stuff to keep him busy and happy, sort of like a miniature Christmas so he wouldn't be upset if me or the baby got extra attention for a little bit: we packed his build a bear "baby" from the hospital class, a new set of crayons and a new coloring book, a couple hot wheels, a small kid version of a comic book, a few snacks, some new pjs and his blanky, and the special present: a fisher price digital camera so he could take pictures of things from his view point).
But even with all that preparation, he was still a little "funny" for the first week that we brought the baby home. He liked my parents' attention, and my husband and I did our best, but he would NOT look at the baby for a week. But then one day he's sitting stoically on the couch pretending the baby isn't by him, watching cartoons, and his baby brother touched his arm and smiled so big. I said "Dude! He knows you're his brother----he's smiling at you!" and I saw Joseph melt. He looked a minute, then his face softened, then he smiled. He's been an amazing big brother ever since! (They have their issues at times some days now that "baby bro" is in the terrible 2s, but that's a different thing).
I think you should REALLY be as open as you can about stuff. Do NOT ask if she wants a brother or sister, or anything like that, that you can't control or deliver on. I think it's great that she won't have to share a room or move. I think you can read those "bringing home baby" or "i'm a big sister" type books from the library and also get some idea of how the child is feeling while reading the books, so you can kind of keep those feelings in mind and be prepared to handle them as they come. Be sensitive to the fact that even though a toddler can be excited about being a big sister, she will also have some trepidation and need to know she's not being replaced and that she is still going to have her cuddle time too! Another thing I think that helped with my son: when the baby went to sleep, I would call him on the couch and cuddle me, read a book, watch a silly show, sing some songs, or something.....but make sure you get your "down time" to rest but also take some of that time to cuddle and recharge both yours and her batteries.
I think the trying to rush the diaper situation could be a mistake. You could rush her into it and then the stress of having a new baby in the house could make her lose that new skill. There's other problems that come into play there. I saw this happen with a little boy I was babysitting: he was 2 and his mom was pregnant. The dad was gung-ho on not having to buy diapers for a baby AND a toddler, so they put a lot of pressure on him and he had some problems. Teach, train, etc but don't pressure them so much, in my opinion.
