Most businesses have a series of steps taken when an account becomes overdue. Usually there is a progression:
1. Friendly reminder
2. In house collection (letters, phone calls). Note that in Pennsylvania there are limits on what can and cannot be done which you will need to understand. Most of the restrictions concern harassment, contacts with third parties, and deceptive conduct (threatening actions which you cannot legally take, or don't intend to take). If you don't understand the rules, you should definitely learn about them before trying to collect the debt yourself.
3. External collection (professional debt collector, or attorney)
4. Suing them.
In all of these cases, you'll have to balance collecting the debt with your desire to keep the client as a customer.
This is intended as some informal comments, not as legal advice.
