My daughter had just turned 3 when my first son was born.
One thing that I saw that no one had suggested is to let her help feed the baby. If you are going to be nursing, concider pumping occationally so that she can still help by giving a bottle now and then.
Definitly let her help with diaper changes. My daughter loved getting the diapers for her brother and I would even let her help squirt the diaper rash ointments onto my finger when it was needed.
The little ones like holding the baby also. I never refused to let my daughter hold the baby (sitting on the sofa next to me) when she asked as long as he was not asleep. Of course being so high energy, it doesn't take them long at all to tire of holding the baby.
I also let her help bathe the baby. During bath time, I would pull up a chair next to me and the sink. I would let her wash him and rinse him with dirrection and guidance. This is her favorite thing to do, and she still helps me do this with my third baby.
Most importantly, when the baby is down for naps, be sure to devote extra time to her and do fun activities with her so she still knows that you love her and haven't forgot about her. Here and there I would also take her and only her out with me when my husband got home from work. A simple trip to the pet store to look at EVERY creature they have was enough to make her day! Now that my middle child is almost 2, I make sure that I alter who goes with me to do simple errands or things once the hubby is home so that they all can have special one on one time with me.