The most effective way to deal with PAS is to maintain frequent contact with the child(ren) and make sure you express your love on a continual basis. Make sure your time with the child(ren) is quality time. When the child is secure in their relationship with you, then the other parent cannot so easily sabotage that relationship. It is also absolutely essential that you carefully DOCUMENT everything so that, if/when you have to go to court, you can demonstrate a pattern of behavior that is not in the child's best interest.
For more information on Parental Alienation syndrome, see the PAS Archive.