Whenever your child does something positive, tell him/her, "We are so proud of you and know when the new baby comes you will help him/her to make good choices."
Inform your child's teachers of your pregnancy, and if they are receptive, the teachers can also make similar statements. Many teachers are very busy, but perhaps if you schedule a conference you both can devise additional ideas to help your child.
If your child enjoys art or reading you can start a "very special baby book." Have your child draw things she/he wants to do with the baby to help Mommy and Daddy. Your child can draw or trace and then tell you in their words what they are doing to help in all of the pictures.
Involve your child in picking the baby's name. She/he will be so thrilled to help to pick out the name for the new baby. Include the choices that you and your partner have decided on, and allow your child to choose from those. Also consider your child's perception. If your child loves the name Lauren, for example, maybe that could be the baby's middle name.
Go through photo albums when she/he was a baby. Reinforce your feelings about him or her as a baby. "You were such a cute baby and so sweet." "I bet the new baby will be just like you!" "Mommy and Daddy can't wait to take new pictures with you and your new sister/brother!"