Draft a letter of formal resignation. Although some states don't require a letter of resignation, providing one protects you legally. Include the date the power of attorney was signed, the full names of the agent and principal, a statement that indicates you're resigning, and the last day you will act as an agent.
Take the letter to a notary public. Sign and date the letter in front of the notary and ask her to notarize your signature.
Make copies of the resignation. You need a copy for your records, copies for any other agents named in the document, and copies for all places where you had the power of attorney on file, such as the principal's bank.
Send the original resignation letter to the principal by certified mail, return receipt requested. Send copies in the same way to all places that had the power of attorney on file and the other agents. Keep your copy and the mail receipts together in a safe place.