I generally recommend that clients keep a folder or box to hold receipts showing the payment of uninsured medical or dental expenses. Then, every three months (for instance, on January 1st, April 1st, July 1st and October 1st), draft a letter to the other parent enclosing copies of the receipts. Here is a template that you can use for the letter. Keep copies of the letters and receipts that you send. This will allow you to easily access a paper trail if you need to pursue reimbursement through the Court with a Complaint for Contempt.
Also keep in mind that, for any non-emergency and extraordinary uninsured expense (orthodontia or cosmetic dentistry is a classic example), you should first notify your ex before the expense is incurred. Most Judgments require this notice and communication to happen before the extraordinary expense is incurred; and, if you fail to give your ex notice before incurring the expense, the court may not force your ex to reimburse you. So, please be prepared to show a paper trail of communication before you spend $5,000 on braces or crowns. Here is a template that you can use when you need to communicate with your ex about an extraordinary uninsured expense.
I hope this is helpful. If you have other questions, feel free to email me directly.