As the continuation bytes always bear the payload in the low 6 bits, Connor Lane Smith suggests writing them out in octal[1]. Though that 3 octets of UTF-8 precisely cover the BMP is also quite convenient and easy to remember (but perhaps don’t use that like MySQL did[2]?..).

[1] http://www.lubutu.com/soso/write-out-unicode-in-octal

[2] https://mathiasbynens.be/notes/mysql-utf8mb4