Don't Lose It
Don't lose your temper with the kids, act too aggressively or harshly criticize the kids. It's likely that if the other parent is attempting to alienate or doing it subconsciously, they are saying that you can't control your temper, are aggressive and don't love the kids as much as they do. If you "lose it", you are proving them right. Keep your cool instead.
No Reverse Psychology
Sometimes the parent being alienated tries to guilt or trick the kids in to wanting to spend time with them. "If you don't want to be with me, then I don't want to spend time with you." Not only does it sound juvenile, it's going to backfire. It plays right into the other parent's hand and shows the kids they are not important. Instead, tell them that you understand that they may not want to come but that spending time with Mom AND Dad are equally important. You're trying to be the best parent you can and by spending more time with them you will only become a better parent and they will start to get a better relationship with you.
Don't Allow the Kids or Other Parent to Dictate Parenting Time
Your child needs to have a set, dependable schedule. Deviations from it leave an opening for the child to perceive unimportance and allows the other parent to take advantage of you. If you wait for the kids to want to be with you, it may never happen. Certainly, if the other parent is attempting to alienate, they are likely not going to ever give up the kids willingly.
Be Positive Not Negative
Spend your time doing positive activities that will evidence your love of your child. You're likely to spend a little time talking and trying to undo some damage but ACTIONS SPEAK LOUDER THAN WORDS. The next time Mom says that Dad doesn't care, your son won't believe it if he can remember that last week you both went outside and threw around the football for an hour or so and had a lot of fun. The criticism is incompatible with his memory.
Don't be Dismissive
Have you ever had someone tell you that you should not feel the way you are feeling? How effective was that? Hey son, you shouldn't be mad at me. Obviously, the child is angry. By dismissing him, your child will think that you don't understand him. Instead, ask him why he is angry. Most children spend a lot of time being talked to and told what to do. If you break the mold and listen to your child and ask questions and are genuinely interested in what he is saying, you're going to be showing him that you care about him and his feelings and are trying to understand him. By listening to him, your relationship will get deeper and the alienation will have a harder time occurring.
Don't accuse your child of merely repeating what Dad said. Accusations are perceived as attacks and, if your child feels attacked, she will feel afraid...of you. If she is repeating what her father said, take the time to listen for a while, then redirect both of you to something positive...see Step 4.
Your child needs a safe haven from the storm. Over time, if you are successful in maintaining a positive relationship with your child, she will learn to recognize that her mother's statements about you are not truth. She will also understand that she cannot change her mother's behavior but she can discard the negativity and hope for it to stop. If you start bad-mouthing, too, then your child has no safe place to land and is caught in both of her parent's conflict.
Seek Professional Help
If you have long-term alienation, you will likely need the help of an attorney and a mental health professional.