It worked for me:

My daughter was the same way. She held a book and turned the pages as I read to her. We also bought new videos that she only watched while I breast-fed. It also helped when my husband spent one-on-one time with her while I was busy with the baby. Sometimes when I was busy with her, and the baby would cry, I'd turn to the baby and say, "I'm helping your sister right now, please wait a minute." This made my oldest daughter feel important and not left out. Since I immediately finished whatever we were doing, it didn't hurt the baby to wait a short but important minute. 

-- B.C. of Grand Forks, N.D.

Develop an environment of normalcy for breastfeeding. Try to do it at a regular time, regular way and regular place, and do not become upset if your preschooler does. Use it as a time to teach her about proper parenting and the needs of newborns. Sometimes, older children feel their own security needs are not being met or recognized. Keep reassuring the preschooler of your love, and provide time to do things exclusively with her. 

-- Keith Dewey of Lafayette, IN

When I was a little girl, my mom brought home a new baby brother. As soon as she stepped in the door with the new baby, she asked me to sit down and placed a pillow on my lap and let me hold him. This made me feel important, and she let me "help" often. If she was feeding him or just wanted to take him for a stroll in the stroller, I was often included and always thought I was a big help. 

-- Christi Becker of Fort Myers, FL

From Jodie:

While the baby is sleeping, talk to your preschooler about the new baby and some of his needs while holding your older daughter in your lap. There are several books on this topic and most hospitals offer a new sibling "before-and-after" class. Show and tell the older daughter how important she is to the newborn. Tape a few favorite songs sung by the preschooler and play them for the newborn. Hang pictures the older child colored on the wall and show the baby how good a job his or her big sister did. Let her pick out a few picture frames for her room and the nursery. Put a picture of the baby in her room and a picture of herself in the baby's room. Make a point to show the children each other's pictures and say things like, "Here's the new baby who needs a big sister's help." After a few weeks, put pictures of the two of them together in new frames. Let the preschooler help pick out the baby's toys and clothes. All these things might alleviate the "left out" feelings.