now my baby is 9 mths and we will teach him what is acceptable behaviour and what is not. he doesn't really remember but after some repetitions he does remember.
for example, we dont't let him flip when we're changing his diaper. we will say "no" and hold his legs down.
he also likes to play with his saliva when eat. and we will pat his lips. sometimes he will get fussy but we will give him a fierce look or speak to him in a fierce tone and he will not do it anymore.
i thnk what's most impt in disciplining is being consistent. if i let my son flip one time and another time i dont't, he will be utterly confused and will tend to through his tantrums more.
as for a new addition to the family, some of my friends bought presents for the elder one when the young one is born. and they tell them that this present is from their younger sibling and that the younger sibling told mummy when he/she was in the tummy. so the older one dont't feel rejected.
i also have a friend who upon hearing his wife is pregnant again let his daughter "adopt" a teddy bear. his daughter carried it, pretend to feed it and "sayang" it. when the younger sibling arrived, she did the same...