Be The Primary Caretaker
The primary caretaker, or the person perceived to be the primary caretaker, will be the one who gets the kids ninety-nine percent of the time. What does primary caretaker mean? It means be the one who cares for the children's basic needs: education, medical, social, emotional, extra-curricular, religious, day-to-day decisions. It doesn't matter if you work full time outside the home. You can still prove you are the primary caretaker if you have a history of handling most or all matters pertaining to the care and welfare of the child(ren).
Be Emotionally, Mentally, and Physically Fit and Healthy
If you are on drugs, alcohol or in some way impaired, you are jeopardizing your chances of getting custody of the children. The court is going to give the children to the parent who seems most able to provide for the child's best interest. Someone who is chronically impaired cannot possibly be fit to handle the child's best interest. Stay healthy. Exercise, eat well, keep psychological and psychiatric issues under control, and avoid the illicit use of alcohol and drugs.
Be Cooperative with The Other Parent
If you cannot work with the other parent in the interest of the children, you will probably be accused of alienating the children and you could lose custody. You may no longer be in love. But you have children together and you have to be the parent best able to foster a healthy relationship with the children and the other parent.
Do Not Relocate without Permission
If you must relocate to another state, you must ask permission from the other parent. Get it in writing. An email will do. If you just remove the child from his jurisdiction, without permission, this could be viewed as kidnapping and you will likely jeopardize your custody.