Examine Iago's methods, and explain why they are so effective.

Iago's methods are devious, despicable, cunning, and utterly mendacious. They are also highly effective. Iago has the advantage over his adversaries in that he's so completely unscrupulous that he'll do anything and say anything to get what he wants. No one else in the play has ever encountered such a dark vision of wickedness before. But though very much the archetypal villain, Iago possesses in abundance the quality of dissimulation, the ability to conceal one's true thoughts and motives. The real Iago—hateful, vicious, and bitter—comes out in his soliloquies. But to those he wishes to destroy, he comes across as a perfectly charming, decent kind of chap, certainly not someone capable of stooping to such extraordinary depths of evil.
Iago is also remarkably flexible in his methods, skillfully adapting them to the person he wishes to manipulate. He is truly a psychologist of genius with a deep understanding of what makes people tick: of their fears, wishes, and desires. He has an unerring knack of identifying a person's weak spot and honing right in to exploit it to his advantage.
For instance, he plays upon his wife Emilia's love and dogged loyalty, getting her to steal Desdemona's handkerchief as part of his sinister plot to destroy her. With the callow young rake Roderigo, he acts the part of Dutch uncle, fooling him into believing that the delectable Desdemona is well within his grasp. And then of course there's Othello, Iago's nominal master. As a deeply insecure man himself, Iago understands all too well what it means to feel as if you don't fit in. Like the Moor, he's an outsider among the social elite of Venice. By slowly drip-feeding poisonous innuendo and insinuation into Othello's unsettled mind, Iago's able to convince him of Desdemona's infidelity despite a complete absence of corroborating evidence.