3 Nephi 18:15

Verily, verily, I say unto you, ye must watch and pray always, lest ye be tempted by the devil, and ye be led away captive by him.

The defenses against temptation and apostasy are watchfulness and prayer. To be on watch is to be on guard, to be vigilant, to be awake and aware and ready. Wise leaders set watchmen on towers so they can spot the enemy while he is yet a long way off. We need to be able to spot the enemy of our souls from a great distance so we can be prepared to mount a defense. If we wait until he is right next to us before we react, we may not be strong enough to fend him off. If we let him whisper in our ear, we may not be able to shut him out. We want to keep him as far away as possible. So we watch. We learn of his tactics, angles of attack, and deceptive maneuvers, and we learn the proper defenses so that when we see him coming, we know what to do. 
 
Nevertheless, even with vigilant faithfulness, we may not be able on our own to counter his every move. Satan has been at this game for a long time, and we are relative new comers to mortality. We need more power and wisdom than we can muster on our own to be victorious, and that is where prayer comes in. Prayer is like binoculars to help us see farther and clearer than our natural eyes alone can see. Prayer gives us intelligence and strength because it puts us in direct contact with the Commander and Chief of our souls. He can make up what we lack on our own and send reinforcements when we are overrun. 
 
We need both our own efforts through faithfulness and God's power through prayer. We cannot do it alone, and the Lord will not do it for us without our contributing effort. With both weapons, we can defeat the enemy and land safely home.