Adding another little set of feet to the family can be a big adjustment for an older child(ren). You want to make sure big sis knows she's still loved, that her role in the family is secure.
What can you do to help prepare her?
Before you make the big announcement to friends and family outside the home, have a special day to talk to her about what's happening. Telling your older child before the rest of the world knows, reminds them that they are important to you. Let her know that she is still a top priority, and you want her to be as involved as she wants (or is capable of depending on age).
There are tons of great simple books out there. Before talking to your child, maybe grab a couple books, and have that as a special gift. Pick out one for her, and make it from the new baby! If your child is of reading age, having a book that is special for her to read to your growing bump is also a simple and easy way to start the sibling bond.
Bring your older child along for baby shopping needs. My older children used to enjoy picking out the new baby's first outfit, and having a say in what the new baby would need.
Find a prenatal class that allows siblings to come along! Explaining the birth process and newborn care can help big sis understand the changes happening in mom, and allow them to have less fear of the big day. Make sure they know the plan for the birth day. Will your older child have a special sitter? Maybe you'll be having a home birth, and they'll be going to hang at grandmas, or even get to take part in some small/big way at home.
What do I do once baby arrives? I don't want jealousy!
When the big day finally arrives, keep your older child in the loop. If you're having a birth center or hospital birth, make sure to let your child know that you'll be gone for a while and that you'll check in when the new baby arrives. If it's possible, have someone bring you're older child to you to visit you when baby born. Maybe even have them be the "first" to hold baby (after you, of course). This will give your older child a sense of pride in their new role as big bro or big sis.
Once baby is born, and everyone is settling in to the routine of having a new baby in the house, create some special time with your older child. There's bound to be a little jealousy, especially when the baby seems to get all the attention. It's normal for your child to feel these emotions, and it's ok to let them have these feelings. By creating special rituals or routines with your oldest, it can help ease these issues. Even small things like story time alone for 5 mins keeps your older child from feeling left out.
Remember that just as this is a new experience for you, it's a new experience for your older child as well. You will grow and bond from all of this!
Who knows, in 20 years they might just be best friends!