B.B.
My son was 19 months when my daughter was born. We spent a lot of time being sure he was a part of the baby prep. We asked him to help choose a blanket for baby, when we were painting her room my husband had him "help". Just little things referencing the baby when we could. We also got a couple fo books from Barnes and Noble about him becoming a big brother, and although I'm not sure how much he retained, they seemed to help a lot, as he heard and saw how the baby would change his home.
After our daughter was born, the first 6 months or so were a little difficult, but nothing drastic, just an adjustment to our routine, as it is with a new baby no matter when you have him or her. We made sure that I got to spend time with J, separate from his sister, so it wasn't as if I abruptly was gone. We had dates, just him and I or my husband and him, and as a family. We tried to include him in feeding the baby, getting her diapers, etc. My biggest heartbreak during all of this time, and the hugest lesson learned, was when she was around 6 months and J was just past 2. She was sitting up on her own, and we were praising her "Good job Maddy!" and so on, the J looked at me with the biggest and saddest eyes and said "Where is my good-job, mommy?". He then turned and went to go play with his blocks. It broke my heart, and I realized that even though we hadn't purposely done it, he was feeling like she was getting all of the attention. We made a concerted effort after that day to be sure to praise both of them.
After that it became significantly easier. Now that my son is almost 5 and my daughter is 3 1/2, it is almost like raising them as twins. My son still gets to do his "big boy" stuff on his own, but he loves teaching Maddy to do things. They play together all of the time, most of the time nicely. They are usually very good about sharing, and helping each other. We still make sure to have special date time with each of them separately, at least a few times a month. They are the best of friends, and I wouldn't have it any other way.
Good Luck!