be patient, with good guidance, you can ease her into receiving the new member of the family happily and slowly the tantrums will subside.
how old is your elder girl?
you can start borrowing books depicting interactions of siblings, new arrival of siblings etc, and even dvds if you can find one (cailou series has, but cailou is a boy and he is expecting a sister).
do not refer to the baby as 'a new baby', that will mean that she is 'old'.
do not always say that she is now older, more sensible, coz she might just want to still be your baby girl.
giving your girl extra attention now might backfired when eventually when the baby is born you will have to divide your time between the two ad then she might feel how come mummy spent lesser time with her. you can dictate a timeslot just for her, and a timeslot 'where mummy needs her rest/or to do her stuff' so that she is familiar that you might at times be away from her.
last but not least, prepare a gift and bring it to the hospital when you deliver. upon your girl's visit, it is preferably that you are not seen in the same room as the baby. put the baby back at the nursery before your girl arrives. get the dad to bring your girl to retrieve the baby from the nursery. when the baby arrives, briefly introduce (do not show too much excitement) and then give your girl the present (books/websites may advise saying that the gift is from the baby to the elder child, but as my elder child is too smart too see thru this trick, i said it is for me to him as a gift to commemorate this occasion) and fuss over her (the elder girl).
hope it helps.