Encourage your child to play and interact with other children in the waiting room. This will help the child feel more comfortable and relaxed as well as making the visit more positive in nature. Usually the doctor will also have a sticker, treat or toy for the child that will also help reinforce the positive.
The internet has made it possible to learn about many different child health issues and even for parents to get the "heads up" on possible diagnosis. Remember that this is not to substitute for actually having your child examined by a trained and certified physician or nurse practitioner.