The UK air dropped matchbooks into enemy lines which contained instructions on how a soldier could fake illnesses to get sent home.
Once the Nazi leaders caught wind of this, they stopped sending their troops home who claimed to have said illnesses. Not only did this get healthy enemy troops sent home, it eventually ended with genuinely ill troops being sent back into combat spreading real disease amongst their ranks.
Scipio Africanus used a great tactic against the Carthaginians at the Battle of Ilipa. The simplified version:
Both the Romans and the Carthaginians had armies composed of their well-trained, homegrown soldiers and not-so reliable Iberian allies, roughly half/half for each. For a few days the two armies were camped close to each other and would come out during the day, form up, and dare the other to attack.
Scipio always put his legionnaires in the centre and positioned his Iberians on the wings. The Carthaginians were like 'that makes sense' and did the same with their army. So they stared at each other like that for a couple of days.
On the day of the battle, Scipio had his men eat well before dawn, get ready and form up outside the camp, but this time he reversed his formation and put the weaker Iberians in the centre and the legionnaires on the wings. Then he signalled for attack, and the surprised Carthaginians ran out of their camps and automatically formed up the way they had the last few days, assuming that Scipio was up to his usual shenanigans.
By the time anyone saw the change in tactics, it was way too late, and the legionnaires tore through the weaker Carthaginian wings and turned on the enemy centre before the Roman centre had even closed with its counterpart. The Carthaginians were all routing and surrounded before their generals could do anything to save the day.
Scipio Africanus went on to be the only general to defeat Hannibal in a land battle.
Cyrus the Great realized that Lydian horses were afraid of camels, so he routed their forces with camel-mounted warriors.
He lighted extra campfires to make his armies seem much larger.