How do I help my 3.5-year-old son to learn that even though he is now not the only child in the family, he is still important?
A difficult task at hand to say the least. I recall a Caillou episode where he was angry because his little sister was getting more attention than he was. (FYI I don't watch that show anymore because Caillou would throw fits and his parents would give in) but the mother eventually reasoned with him that although her attention is to take care of his little sister she gets to do more grown up (big boy) things that his sister doesn't get to do. I took away from it that children usually want what the other has at that very moment, and instead of focusing on stopping that behavior shift their attention on what they have.
I find that boys want to be protective. They have strength and they know it. Put the child on look out duty of the dog/cat so that the animal doesn't bother the newborn. Have the child build a pillow moat around the new born. We tought both our newborns sign language, so it was natural to let older grab the hands of the younger to show them how to sign something instead of us doing it. (it's kind of funny watching my older daughter force her younger brother to sign sorry). Put a boppy around the older son and lay the newborn on his lap. My daughter would stare at her brothers face and coo and hold his hand. Bonding is important for parent/child as well as siblings.
That's my two cents. Now that my kids are a little older I don't get many complaints about us spending more/less time with the other.
This might sound odd, but get your son a doll (girl preferably - to mirror the baby sister), and encourage him to take care of it. This is better done before the sister arrives, but it will still have a lot of benefits, and can give him something to do in parallel while you give attention to the baby.
We have a baby (we think its a girl) on the way, and have told our son (3) as soon as we told anyone else, and show him pictures ("this is what your sister/brother looks like right now"...) and talk about what it will be like when she/he gets here.
He also has, and has had for a while, a baby boy doll (in addition to the dozen other stuffed animals, though the doll is treated differently), which he carries carefully up the stairs, protects from zombies, and covers with things to keep him safe and warm. I think this is one of the best ways to engage a young child in such activities, since it is relatively easy, and can make him/her feel very important. It can also serve as a replacement behavior for attention-seeking when you need to attend to the infant: "while I change your sister, can you show me how you change your baby?" kind of engagement.
Show him that his sister needs him. If she cries let him calm her down. If she looses a toy let him bring it to her. Let him push the buggy.
At the end he will know that he is the most important person in her life: He actually is.