N.J.
My two boys are 22 months apart.
To prepare my first son, we let him rub mommy's belly and tell him there is a baby brother in there who will be here soon, just like when you were first born. Then when we got things for the new baby (clothes) we would put it on my belly and asked him if he liked his new brothers outfit...
we got him books on being a big brother and he helped set up his baby brothers cradle...and we waited and waited.
kids (even at 18 months) can understand more than we give them credit for, I would share with him as much as you possibly can.
Once the new baby was here, when he saw his baby brother for the first time, we made sure it was only us in the room (me, the father, new baby and our first DS)...when we had visitors come, we told them in advance to dote on the first son when they walked through the door, then the new baby...taking the attention off the new baby, this really helped in him not feeling pushed aside. We had our guests do this for about two weeks, also if they brought the new baby a gift, to bring the older son a small gift too (a matchbox car was usually the gift he got, but the kid loves cars! This way he never felt pushed aside, and I never really had any problems with him acting out to get attention.
congratulations!