I was amused by the suggestion of sending a secret message written on shaven head of messenger, who then could not be sent into enemy territory until his hair had re-grown. By that time the message would be a bit stale, don't you think?
I also think the idea of reducing size of message by repeated use of photocopier is hardly practical. The resolution of a photocopier is inadequate for that job.
I remember a scheme that involved dyeing a thread with bands in Morse code or soemthing like that, and then knitting it into part of a garment.
AnnaStrophic ought to be able to describe other methods. Below is what wordsmith wrote, and something i found on Internet.

steganography (ste-GUH-nog-ruh-fee) noun

Secret communication by hiding the existence of message.

A couple of examples of steganography: shrinking the
secret text (by repeated use of a photocopy machine) until
it's the size of a dot and then putting it in an unsuspected
place, such as on top of a letter i in some innocuous letter.
Second, shaving the head of a man, writing the secret
message on his pate with unwashable ink, and then letting
the hair grow back before dispatching him to the
destination. To take an example from modern digital
techniques, one could put the text of a message in the
blank spaces in an image file.

The history of cryptography crackles with famous names.
Shifting the whole alphabet forward or backward by one or
more letters, so that, for example, A becomes B, B becomes
C, and so on, is known as a `Caesar shift', one of the
simplest kinds of cypher or letter-substitution code (see
the title of this article). Julius Caesar also used the ruse of
writing a Latin message in Greek characters so that it would
be unreadable if intercepted by the Gauls. One such missive
was delivered to the besieged Cicero fixed to a spear which
was hurled into his camp by a messenger. And pin-pricking
the letters of an existing document to spell out a secret
message, a form of steganography popular in Victorian
times when newspapers could be sent by post for free,
dates back to Aeneas." Moreover: Tijguz cvtjoftt, The
Economist (London) Aug 28, 1999.
[From Greek stego- (cover) + -graphy (writing).]

Steganography simply takes one piece of information and hides it within
another. Computer files (images, sounds recordings, even disks) contain
unused or insignificant areas of data. Steganography takes advantage of
these areas, replacing them with information (encrypted mail, for
instance). The files can then be exchanged without anyone knowing
what really lies inside of them. An image of the space shuttle landing
might contain a private letter to a friend. A recording of a short sentence
might contain your company's plans for a secret new product.
Steganography can also be used to place a hidden "trademark" in
images, music, and software, a technique referred to as watermarking.