Don't lose any units. No body bags=no war weariness. Use mobile units such as Horsemen, Knights, Calvary, Tanks. Back them up with catapults, cannons or air power. Treat each unit as a precious son. Go to great lengths to save one unit. Feel bad if you take two casualities taking out a stack of ten opponents. Do all of this your army will gain the aura of invincibility and will have very little war weariness.