In your town you might have a property tax consultant. Or you can ask a real estate agent to refer you to one. SE, Washington, DC 20003. That's why it's so important to prepare before a meeting at your assessor's office or appearance before a tax board. Keep monitoring your assessment and tax statements, though.
Mistakes can crop up again.
You can protest on your own or join a tax revolt in your community. Both will require home work on your part. The tax records you will be looking for are at your assessor's office and are open to anyone. Be sure you are comparing your home with one that is almost exactly like yours and in the same neighborhood. You might find it more practical to hire someone to pursue this for you.