S.D.
This is a really stressful time for everyone and I can't imagine having a premie to boot!
We had the most success with positive reinforcement.
What helped with my kids was playing up the "special" time my son was spending with my husband (and thankfully the in-laws and my parents) over that first month. We'd say, "You and daddy are going to have a special play date" or "You are going to have a special dinner with Grandma and Grandpa." The other thing we did was ignore the negative behavior and play-up how hurt the baby must be that the brother hit/bit her. This helped my son realize that his aggression towards the baby didn't have the outcome he wanted (i.e. it didn't make us give him more attention). We also involved my son in big brother activities like helping get diapers and wipes for the baby or entertaining the baby while i changed her. He appreciated having responsibility and being included in the baby activities. We also let our son indulge in his regression a bit - he hadn't had a pacifier in 4 months when the baby came home with one. He took it and (didn't know how to use it) but tried to use it. This reignited a habit that lasted ~2 mos. We just ignored it, and mentioned every once in a while that pacifiers are for babies. Finally, I think what helped the most was how much we hammed up the fact that the baby was interested/loved/adored/etc my son. Any time my son was near, we'd play up how interested the baby was in him, and how she was watching him and really liked him... all of this starting of course long before the baby noticed him, but within a few weeks it was actually true - the baby found my son a lot more interesting than us. My son may be a sucker, but who doesn't love someone who loves them?
One last thing - once my daughter was a little bit older, I traded kids with my husband and had my own special shopping trips, park dates, cuddling with my son. My son just seemed to need help understanding what this new baby meant for his relationship with the family.
I expect you are feeling or will feel is guilt about having the second baby. In my case, I never questioned that I'd have two kids, and I always strongly believed it was best for everyone, but the hormones kicked in after the baby was born and I felt SO guilty for changing my relationship with my son, and for making him feel like "the other woman". My pediatrician reinforced all the things I knew by echoing that having a sibling is really good for your son - it helps him learn to nurture others, lead others, and gives him a friend for life. Chances are good that your son will be the first to make your daughter smile, and he will likely elicit the greatest laughs from her. Now 1.5 years from where you are, now, my kids often cuddle on the sofa during book time, and my son very warmly kisses and hugs my daughter goodnight.