Personally for me, we have always wanted 3 chidlren so since day one with Flynn I have had that at the back of my mind. I adore the little guy, but even as a little bub if I was in the middle of going to the loo or something I couldn't stop and he'd cry I'd say to him (and myself) "mate you are gonna have to get used to waiting, cause one day you will have a brother or sister. Not that he listened to me of course! But it helped me get used to the idea. Maybe you could try doing similar things?
I am also reading a good book at the moment about these sorts of issues and the dilemmas of loving more than one - it is called "Three Shoes, One Sock and No Hairbrush - everything you need to know about having your second child". You can get it pretty easily on Amazon and I do recommend it.