If you can hand off the document in person, that's ideal. Otherwise, one option is to use a free online service to encrypt your completed Form W-9 and email it securely to the requester. You could also use an encrypted file-sharing service. Another option is to send it through a service such as FedEx, UPS, or the postal service, though there is no guarantee your form will not be lost, stolen, or tampered with in transit. There's also no guarantee that the recipient will store the form securely even if you send it securely, so ask about their method of store prior to sending.