1) The inevitability of interception- Sooner or later, encrypted messages will be captured.
2) The cost of decryption- Decryption is inevitable. All the coder can do is increase the cost (time, money and computation) to the enemy
3) Signal/Noise- Introducing multiple algorithms to create random noise messages can increase the cost of decryption but reduce the probability of interception. The technologically weaker player should introduce more noise to distort the signal/noise ratio knowing the messages are being intercepted anyways ( especially electronic, radio or digital)
4) Intercepted flag- Interception takes time. Flags to capture interception shall help coders in knowing which messages have been intercepted and which not. This of course can be manipulated by the interceptor.
6) Kill all the decryptographers- Focusing on the personnel of the enemy can help increase the cost of decryption.
(yawns and shrugs)