If possible, get counseling for your child, preferably with a therapist trained to recognize and treat parental alienation syndrome. If it's not possible to get your child into counseling, go to counseling yourself to learn how to react to and counteract the problem.
Don't do anything to violate any court orders or otherwise be an undesirable parent. Pay your child support on time and fulfill all your parenting obligations to the letter.
Don't react to the alienating behavior by engaging in alienating behavior toward your ex. This just makes things worse and further harms the child.
If you are not getting court-ordered time with your child, go back to court and ask that the parent violating the court order be held in contempt of court. The sooner the court knows about the violation of the court order, the more likely it is that the problem can be stopped before it becomes permanent and irreversible. If your custody order is not specific as to exact times and dates you are to be with the child, ask the court to make the order very specific so that there can be no doubt what is required.
Try not to blame your child. Your child did not create the situation, and desperately needs your love and affection. (Next)