Rule 318. Designation of agent for service. A person may be
designated by a natural person, corporation or partnership as an agent
for service in a writing, executed and acknowledged in the same manner
as a deed, with the consent of the agent endorsed thereon. The writing
shall be filed in the office of the clerk of the county in which the
principal to be served resides or has its principal office. The
designation shall remain in effect for three years from such filing
unless it has been revoked by the filing of a revocation, or by the
death, judicial declaration of incompetency or legal termination of the
agent or principal.