1) Children want to love and care for the other parent without feeling guilty. It will make a huge difference if you show enjoyment in their connection with the other parent.
2) Make hand-offs of your children to the other parent as pleasant and relaxed as possible. This is not the time to bring up sore topics or to criticize each other for bad decisions. If disagreement ensues, take it away from all earshot and visuals of the children. Sometimes bringing a friend or relative with you helps to relieve the desire to pursue such conversations. There are also centers that have been designed to help with the exchange without either parent having to see each other. In my opinion, this is a last resort as the children need to see that their parents can act like mature adults. If one of you cannot then this center is a great option.
3) Allow your child time to adjust to being back home with you. They may act out or become emotional over leaving the other parent and feeling guilt about feeling disloyal to you. If their visit wasn't enjoyable, they may feel upset. Allow them to talk about all feelings with you in a safe environment and never badmouth the other parent. Be sure to seek professional help if your child is not adjusting to being home after a few days.
4) Do not for any reason use your children to spy on your ex. I cannot stress this enough. Children already feel guilt for living with one parent and visiting another. They feel guilt for not being able to share all of their great moments with both parents. Your child is not a sounding board for what your ex is doing.
5) Understand that your children may misbehave when they come back or while they are gone. It is important to be on the same page with the other parent on disciplinary actions and consequences so that there is a common ground on what is expected. I realize that in my situation as in many others, parents rarely see eye-to-eye on this topic. It is important that you uphold all of your standards without giving in to the guilt that comes with a child moving from parent to parent. It may feel like an endless battle, but it will pay off in the end. Your child feels secure within boundaries and by giving them these boundaries you are showing them how much you care for them.
If there is little or no contact with the other parent it is your responsibility to educate your children on how the other parent fits into their lives. You only get one chance to do this right. Make it count.