Common methods for hiding the fact that a message was communicated (steganography)
Miniaturization (low profile)
Drowning -- hiding a needle in a haystack
Or a combination thereto.
Coercion is the brutish way of scaring nosy people off the communication lines so they don't check for passing messages. Miniaturization is often in a physical sense, compressing a message in a micro code to a dot mark that may go on any piece of mail, etc. Drowning is the means to surround the hidden message with so much similar stuff that it is not visible. Diversion is where one directs the adversary's attention elsewhere.
It is noteworthy that steganography may only help any cryptographic measures used. It is much better that your adversary is not even aware that a secret is passed around. By using the two strategies we acquire a fall back position in case our steganographic measures fail.