1. Rely upon the ignorance of the masses.
One of the most common critiques I hear about conspiracy theories is that the government would have to be masterminds to pull them off, and we can clearly see that they're a pack of fools. Well, that's misleading. Most cospiracies are done POORLY. They make numerous mistakes and obvious oversights. The official story is always full of holes and almost zero investigation takes place. The truth is there for anyone who cares to see it. -- Yes. They are a pack of fools. But they rely on us being an even larger pack of fools, and by and large, we are.
2. Control the media.
Check. -- If an "authority" figure tells someone what to believe or do, most people will comply. See the Milgram experiment.
3. Attach a stigma to contrary thought.
When it comes to the multitude of conspiracies that have existed, there are tons of whistleblowers and truthtellers. They are simply and automatically labelled as "crazy" or "conspiracy theorists". And on the other hand, people who might be inclined to come forward with information are discouraged by the fact that they will be placed into those social categories.
Each person involved with the execution of a conspiracy only has a small part of the puzzle. They follow simple orders without being given enough information to make connections.
5. Control the top.
The groups of men who orchestrate a conspiracy and know most of the details are highly unlikely to come forward. The outcome of the conspiracy usually benefits them financially, and admitting your place within a cabal is social suicide. Also, this upper eschelon of controllers have policies of Mutually Assured Defamation, or consentual blackmail. It usually involves child sex trafficking. Watch "A Conspiracy of Silence".
6. Destroy liabilities.
Any low-level player in a conspiracy is terminated. The government mostly uses soldiers for crucial tasks. They are conscripted into "Top Secret" programs. Once the job is done, to prevent the soldiers from making connections and possibly coming forward, they are killed. To their family and the world they are considered KIA in whatever random war is being waged at the time. It is a simple process.
As to the "who" of 9/11, please watch this video:
I hope this answered some of your questions.