All warfare is based on deception. Hence, when able to attack, we must seem unable;
when using our forces, we must seem inactive;
when we are near, we must make the enemy believe we are far away; when far away, we must make him believe we are near. Hold out baits to entice the enemy. Feign disorder, and crush him.
If he is secure at all points, be prepared for him.
If he is in superior strength, evade him.
If your opponent is of choleric temper, seek to irritate him.
Pretend to be weak, that he may grow arrogant.
If he is taking his ease, give him no rest.
If his forces are united, separate them.
Attack him where he is unprepared, appear where you are not expected. These military devices, leading to victory, must not be divulged beforehand.
From: 孫子兵法 – The Art of War, Sun Tzu