"And it came to pass that when Jesus had spoken these words unto his Disciples, he turned again unto the multitude and said unto them: Behold, verily, verily, I say unto you, ye must watch and pray always lest ye enter into temptation; for Satan desireth to have you, that he may sift you as wheat."
The image of Satan "sifting as wheat" is interesting for several reasons. One involves the early claim, now discredited, that the ancient Nephite civilization did not cultivate wheat. Under that argument they would not have understood the analogy. However, once stores of ancient wheat and barley were discovered, the criticism was debunked. There's a BYU article on this by Robert R. Bennett titled "Barley and Wheat in the Book of Mormon."
Satan "desires to have you." You are wanted. Not because he has your best interest in mind, but because he wants control. He wants to gain power over others, limit their choices, and make them his slaves.
The manner wheat was sifted was to use a sieve to separate grain from husks, tares, stones and other chaff. The wheat would be kept, the refuse tossed into a pile to be discarded. Sifting was vigorous and tossed the grain about to separate it. This suggests being completely under Satan's control, being tossed about, and being discarded. It is a horrifying image, because the result would be domination by the adversary of your soul.
Satan's great desire has always been to separate men from their agency. He seeks to enslave those who fall under his power. Using wickedness, appetites of the flesh, drug dependence or other addictions, the end goal is always the same. He seeks control. He craves the god-like power to have dominion over others. Since he forfeited any right to gain power in a godly way, he seeks now ungodly power through coercion and compulsion.
Whenever you find compulsion, dominion, control, or force being employed, you have found Satan. (D&C 121: 37-41.) He wants to cut you off from heaven, and uses control to limit access to the heavens. When people voluntarily surrender their responsibility to follow the Lord, Satan has acquired by persuasion what he craves to acquire through force.
The antidote for falling under Satan's control is to "watch and pray always." Why watch? Why "pray always?"
To watch is to be observant and detect elements of control, dominion and compulsion. It is to become vigilant in separating the will of men from the will of God. It is to keep the Lord's teachings in mind, and to measure any person's teachings, actions and persuasions against the standard the Lord has explained.
To "pray always" is to retain a personal connection with heaven. Particularly, to retain that connection through the Holy Ghost, and through Christ's Spirit, you seek to always have with you. If this is a lively connection, you are able to avoid being "sifted." If it lapses into darkness, you are vulnerable to being taken captive.
These are simple expressions anyone can understand. It is not the difficulty of the teaching, but the difficulty of the implementation which keeps people bound in darkness. Traditions, widespread acceptance of false ideas, excuses for failure, and rationalizations for why things are as they are, all prevent us from reading these teachings with the eyes of a child. The Lord's teachings are distorted even as they are being read by the blinders we wear. When the eye is filled with darkness, how great is the darkness within. Christ spoke about that in the previous sermon.