Whether divorced or never married, if the other parent is interfering with your rights to parent and be with your children there are some general strategies to employ so that if you do make an application to change custody you can show that you have been doing the right things. Since parents and children are different I can only make general suggestions many of which will be helpful and some may not apply to your situation.
1. Exercise all of your scheduled parenting time. (If it is being denied, then keep a calendar of the interference, keep copies of text messages and emails, request make-up time, consider whether to make police reports for documentation although that can be a very delicate thing to do as some children will negatively react if the police are called by one parent on the other.)
2. Get on the school and extra-curricular calendars and portals so that you can follow and attend extra-curricular activities and school events and parent/teacher conferences.
3. Send an email to the teacher letting the teacher know that you are involved and want to know the good and not so good about how your child is doing in school. Let the teacher know that you are a resource. If you are not able to get on the portal ask the teacher to copy you on emails. This should also be done with team coaches, gymnastics, dance, music and craft classes.
4. Contact the children's doctors and let them know that you are involved and concerned. Ask for portal access so you can see reports and test results. Let the doctors know that you are a resource and want to be consulted about medical treatment.
5. Let your kids know you that they can call you, FaceTime, text or email anytime they want. If they are not old enough to do it on their own explain to them that they can ask the custodial parent to assist.
6. Bring the children to your family events for grandparents, aunts, uncles, cousins and half siblings. Get them involved with your family.
7. Make parenting time productive whether it is doing schoolwork, recreation or just hanging out together. The goal is to create an atmosphere where the children want to come rather than to choose other activities that are more fun than being with you.
8. Be flexible. If your children ask to reschedule because they have something that is important to them consider whether you can agree and schedule a make-up.
9. Be careful with correcting behavior issues. Of course bad behavior has to be addressed but consider what the punishment is for the inappropriate behavior. Understand that taking away a cellphone or taking away internet access is viewed by most children as if you took away their oxygen. They believe those devices are their lifelines and as soon as one parent takes it away they want to live with the other parent or not see the parent that put the restriction in place.
10. Do not use the children as messengers to the other parent. Do not disparage the other parent to the children. Do not make the children choose which parent to love. Let the children express their feelings to you.
There are "Bill Of Rights" for children that can be found online. Likewise, non-custodial parents have rights and resources for them can also be read online and be a source of guidance. Most parents if they pause before they react can often make the right decision. If you are forced to deal with the custodial parent from hell, then document as much as you can and keep showing the children that you care and want to see them. Then consult with an experienced matrimonial attorney for guidance as to what else you should do (if anything) before seeking to enforce your rights or seeking a custody change.