Hi Mamaesi,
Congratulations on your pregnancy. When are you due?
Ds will be 19m when DC2 arrives in sept so I can tell you what I am doing in preparation for blowing his world apart... but can't tell you wether it works or not!
The main thing I guess is talking about babies. We look at them in books/rl and he knows what they are.
When the new baby is more obviously moving inside me, and he can see and feel it, I plan on telling him that it is a baby, and that it will come out and live with us one day. It's impossible to know how much he'll understand, but he often surprises me in this regard and it can't hurt!
We have worked on "being gentle". We stroke the dog gently and say, "oh Ds, you are being very gentle...doggy likes it" Same with being gentle with people, so he really gets an idea of what it is to be gentle and hopefully will be able to be gentle with the new baby. 
Ds is still bf so I've been pretending to bf his toys, so he gets used to the idea of sharing his milk. He finds this hilarious and brings me things to bf, odd stuff like the telly remote and some kitchen tongs 
We also do taking turns...like if we all share a snack Ds will have a turn, then DH, then DM...and back to DS. We narrate it using the word "turn" a lot. Hopefully when the new baby comes he will understand that if it is the new babies turn to, say be held, he will get a turn after...but I must remember to come good on this one for him!
Plan on using a sling a lot...
Erm that's all I can think of for now but am watching with interest.