Make sure you are well-informed on the topic of sexual abuse. Know what is going on with your child at all times. Know who are his friends, his teachers, etc. Listen to your child who might be feeling ashamed to talk about the abuse. Do not accept his/her short answers too quickly.
Children who are not properly informed about sex are at greater risk for sexual abuse. Educate your child about sex, even though you may not feel comfortable about it. A good way to start with the subject is while you are teaching your child the names of body parts.
Teach your child to be respectful and obedient towards adults. However, let him/her know clearly that respect does not mean that he/she should engage in activities that you disapprove of just because an adult told him/her to do so.
If you are not present, make sure your child knows exactly what to do. Teach him/her how to get away from a situation that does not feel "right", and to come to you immediately. Have training sessions about this with your child regularly and get the rest of family involved also. Most often, sexual abuse to children happens inside the family circle.