Books are a great way to help your little ones prepare for kindergarten. When my girls were little, and they finished preschool, we spent some of our summer months reading books about kindergarten. It helped them to get excited about going and also gave them an idea of what to expect.
If your child has any particular concerns, you can usually find these within a kindergarten book (like riding the bus or finding their teacher at school), or you can try looking for a bus about that specific thing.
Other books, like The Kissing Hand, can help provide kids with coping mechanisms for when they miss their parents.