Congratulations! I am an only child and now a mother of three boys and I was VERY anxious about going from one to two kids. . . my parents had no advice and I was concerned I would be surrendering the special relationship I had with my first son when the next one arrived. Rest assured, love really does grow, as corny as it sounds. This new baby will allow you to love your first in whole new ways as you see her as your daughter AND as a sister.
Some things I have loved are using a baby sling and an Arm's Reach Co-Sleeper. Both are sanity savers when you're trying to juggle things, like taking a walk with a toddler and a little baby, or, say, sleeping and nighttime parenting. Could not do it without those pieces of equipment. We got my oldest boy a small sling to carry stuffed animals in when I carried his younger brother in my real baby sling on walks. Too cute and it made him feel like he was doing what I was doing. (I like to hope my daughter-in-law will thank me someday for raising him to be a good dad.)
Also, we loved the books by Jeanne Ashbe. She has done two lift-the-flap-type books appropriate for small children about to become siblings. They are called "What's Inside?" and "And After That." Both are fabulous and so cute, and address in a very simple way how a baby is growing inside mom, and how they will not be neglected even when things change. I also like Dr. William and Martha Sears' books "What Baby Needs" and their other children's library books. There are a lot more words than the Ashbe books but great pictures so you can talk about the topics on several levels.
When I was expecting my second, I found an amazing midwife who let my older boy be a part of nearly every prenatal visit. He was able to hold the stethoscope to my belly (while my midwife used her $$$ doptone) and "check" on his growing brother. I had my second son at a freestanding birth center and my first son was in the next room watching a movie ("Pinocchio," I think) and it was truly beautiful that he was along for the ride there, and we were a complete new family on the way home. Everyone has to do what works for them and I'm not knocking anyone else's choices, but for my family, I really credit that totally together start with giving my little men a feeling of attachment and brotherhood from the get-go. My first has felt a sweet obligation to help his younger brother since his birth, by bringing me diapers and burp cloths, and that feeling continues today at age 6 (as he bosses the three-year-old to find his shoes and get in the car). ;) If you are experiencing a healthy, low-risk pregnancy, I highly recommend finding an experienced, family-centered midwife who can offer you personalized pregnancy care and an opportunity to include your older child as much as you're comfortable with. I experienced gratitude for an excellent midwife again this August as I welcomed my third son--this time I skipped the driving part and welcomed him at home, with his brothers watching a movie downstairs and coming in to kiss their new little brother right after he was born. They are sweetly devoted to him already. Perhaps consider an out-of-hospital birth if your pregnancy is normal--for me, the recovery time was much shorter and the joy in my family much increased.
My biggest adjustment with a growing family has been figuring out how to cook and eat--a stockpile of easy and pre-made stuff in the freezer and pantry was a real lifeline. And as much as I want family togetherness right after the birth, it was a relief to have my grandma stay and be my postpartum doula and help and play with my oldest child when my hubby had to go back to work. Remember, somebody's got to "mother" the mother as you settle in. Again, I was so glad I wasn't solely in charge of everyone for the first couple weeks, but I was also glad to have help come stay with us so my oldest child didn't have to go away for me to get some rest and he could also settle in to his new brotherhood right at home.
I kept a few small items in a basket by my nursing chair for awhile so I could still read or play with stickers or play a little game with my oldest while I nursed his brother. I saved then for when I needed them only, so they had some novelty.
Finally, we have "dates" with the children and that started when my second child came. I started saying, "Hey, will you go with me on a date?" to my oldest when my husband could stay with the baby. Then we'd just go to the grocery store for half an hour, but we'd chat in the car and I'd let him pick out some Popsicles or fruit or whatever at the store and make a point to pay attention just to him. We both loved it and I've tried to continue that, sometimes with a real fun date and sometimes with an errand I've tried to make fun. One--on-one attention is important, I think.
My sincerest best wishes to you!