Maintaining registration calls for both administrative and executive action.
Administratively, you need to file with the USPTO a showing that you’re still using the mark after 5 years of the registration, and every 10 years, you need to file a request for renewal.
On the business side, you must continue using your mark at all times, even after the registration. This requirement is often overlooked because your staff who handles trademark typically has no saying in continuing/discontinuing a product line.
You need to monitor your registration status and enforce your trademark rights yourself.
Trademark Status and Document Retrieval (TSDR) service offered by the USPTO is a good place to start. The USPTO recommends every trademark applicant to check TSDR regularly.
The USPTO does not enforce registered trademarks for the owners. It’s your obligation to monitor infringing activities in the market and take actions to stop them. Often it involves writing a letter to notify the wrongdoer and ask for compliance/compensation, but you may need to bring a lawsuit if there’s a dispute.