I had my second son when my first was 18 mths old.....this is what helped us. I gave my son a baby doll complete with blanket and bottle about a month before hand. We took that doll EVERYWHERE with us :) I even considered putting the baby's car seat in the car, but thought I'd look crazy ha ha. I made sure that we fed the baby daily (at least once) and put the baby doll to bed, wrapped in a blanket, etc. We made sure the baby was fed, etc. So it sort of got him used to having someone else around to take care of. I also talked a lot to him about when new baby came and used my new son's name a lot to get him familiar with him.
When my youngest was born, we did a lot of the same routines with the baby doll that we did with the new baby. so when I nursed my son, my older son fed his baby a bottle.....it eventually got fazed out (the doll, not my younger son) and my older son began to help me with his baby brother. I also made sure that when baby brother was sleeping, big brother and I did something together. Watched a show, read a book, made lunch together, etc. It helped him realize he was still important.
Oh and the main thing I read somewhere that really struck a cord and I tried to use to the best of my ability. Was if Big Brother and I were doing something and baby brother started to cry. I didn't immediatly stop doing the activity. I made sure to finish what BB and I were doing and explaining that when we were done, I needed to help little brother cause he was crying. Most of the time BB wanted to stop before I did to go get little brother, but on those times when he didn't. I would like to think it showed him that he was just as important as little brother.
good luck, two boys are soooooooooooooo fun. they are like best friends built in to the same house.
I wouldn't trade it for the world
K.