I build up to it. Just starting with little hints here and there, "what if we had another..." "do you remember when <insert names> was a baby.....I miss that" etc. Inserting sentences like that as often as possible into coversation, but where there is little or no room for him to replie to it. It gives him time to become accustomed to the idea that you might have changed your mind, given him time to think about what having another baby means to him, and then doesn't come as such a shock when you do come out and say "hey, I want another...."
I have done this a couple of times so far [and if I am being completely honest, have already started 'hinting' at maybe having another... ]