1. Allowing your new spouse to act as the go-between with other the other parent. In a custody situation is it important that the parents communicate with each other. Your new spouse or significant other should not be the one to handle all the affairs of your child. It is important as a parent to show that you are involved with your child.
2. Withholding the children unless they are being seriously abused. It is always a bad idea to take unilateral action and instead allow the courts to decide what contact is necessary. Withholding children without a very valid reason will end up as a factor against that parent in a custody proceeding.
3. Making unilateral decisions about the child without the other parent. In most cases, parents have joint legal custody and both parents are entitled to attend doctor's appointments, make health care decisions, decide on schooling for the child. Withdrawing a child from school without consent of the other parent will often result in contempt. Not working with the other parent to schedule doctor appointments they can both attend can also result in contempt and can harm a parent in a custody dispute.
4. Posting photos on Face book that can be used against you. Be careful what you post on the Internet as it can come into play in a custody case.
5. Keeping the children from grandparents. If you exclude grandparents completely, oftentimes you will end up with additional litigation. Grandparents have rights and can bring their own action for visitation.
For more information see: http://www.ulmerlaw.com/Family-Law-Divorce/Child-Custody-Visitation.shtml