History with the Kids
Since courts typically decide custody cases under the "best interests of the child" analysis, they may be more likely to favor the parent who can show a history of parental involvement. The judge needs to know which party has been the one getting the kids ready for school, helping with homework, taking off work when they're sick and performing other important parenting tasks. It relates not only to what the children are used to, but it also implies a silent agreement by the other side that, historically, that party has been an appropriate person to do these things.
Lifestyles and Work Schedules
Courts can have a difficult time figuring out how to place children between two equally fit parents with a history of substantial involvement. Because of this, demonstrating availability and willingness to take care of the kids can work in a parent's favor. It doesn't make sense to award equal time to a party who works sixty hours a week and simply isn't there. The judge needs to know where each parent lives, who the parents live with and whether each parent can provide a fit and safe living environment and proper supervision for the children.
Circumstances of the Children
Just like no two snowflakes are alike, no family is exactly the same. Some children may have special needs or circumstances that can influence how a judge rules on placement. Kids' medical, mental or emotional disorders may suggest they need more placement time with the parent who has the easier work schedule and thus, greater flexibility. In a motion to change custody, the children's performance and well-being in their current school district may also be relevant. The parent defending an existing order may want to show how well the kids are doing, and the attacking parent might want to show how poorly they are doing.
Specific Instances of Negatives and Positives
While parents should focus their efforts on building themselves up rather than tearing the other side down, they may want to point out major examples of the other's inability to parent the children well. Substance abuse, instability and mental illnesses all relate to parental fitness. Rude and vindictive behavior, especially in the presence of the children, can also cast a poor light on the other side. Specific negatives about the other side can be used to highlight positives about a party's case; being stable where the other is unstable and forgiving where the other is vengeful. A parent's whole case rests upon showing the court that she is an adult who can and should take care of the children; sometimes, this involves showing that the other side can't.