I bought my son the book, "The Kissing Hand." This a great children's book about going to school for the first time and missing his mommy.
Help your child make a friend in the beginning. Encourage them to introduce themselves and ask if they want to sit by each other or play together.
If anyway possible, you and your child can set a time for you to come to preschool and say hello, read a story to the children or just have lunch. Let him/her know that you cannot do this all of the time but that you miss him/her too and you would like to see him/her at their school, when you can.
Introduce yourself to a few of the parents and set up a after school playdate for the children. Meet with a mother or two and allow the kids to play together out of the school setting.