My son was 21 months when our daughter was born. As a pre-empt, we got him a baby doll and talked about the baby in mommy's tummy. He wasn't very verbal at the time, but sometimes he would say baby and touch my belly. I would also ask him where's the baby, and he'd point to my belly. I didn't think he really had a clue. When my parents brought him to the hospital after she was born, he was very excited - more to see me it seemed than to see her, but he was very hyper! I spent some alone time with him before we brought her in, but from the moment he saw her, he never again pointed to my belly for the baby. If we'd ask him, he'd point to her. He didn't use the baby doll a whole lot before or after, but I think it helped with the concept. We got one of those that's about the size and weight of a newborn.
We also did the big brother T-shirt - most hospital gift shops have these. He loved wearing it.
Good luck, and enjoy the last days you have one on one with your little one. Things will never be the same.