The subpoena shall be served in the manner requested by the applicant, by either (1) personally serving a copy or (2) mailing a copy by registered or certified mail, return receipt requested, not less than six days before the day of the hearing or deposition which the witness is required to attend. The person making such service shall make a return thereof showing the manner of service. A subpoena may be served by any person not interested in the matter or by the sheriff. When served by any person other than a sheriff, proof of service shall be shown by affidavit, but no costs of serving shall be allowed, except when served by a sheriff.