• Mental discipline (Samadhi)

6) Right effort (samma vayama)
Right effort consists of the Buddhist practitioner's continuous effort to, essentially, keep his or her mind free of thoughts that might impair his or her ability to realize or put into practice the other elements of the Noble Eightfold Path.


First of all, although Liber Librae is an adaptation of Golden Dawn teachings, it explains this idea quite clearly which needs no comment:
16. To obtain Magical Power, learn to control thought; admit only those ideas that are in harmony with the end desired, and not every stray and contradictory Idea that presents itself.
17. Fixed thought is a means to an end. Therefore pay attention to the power of silent thought and meditation. The material act is but the outward expression of thy thought, and therefore hath it been said that "the thought of foolishness is sin." Thought is the commencement of action, and if a chance thought can produce much effect, what cannot fixed thought do?

Elimination of superfluous thoughts/actions
In Thelema it is also understood that one has to eliminate actions, words, and thoughts, but this isn't done to free the self of impairments of the realization/practice of the Eightfold Path. These superfluous thoughts are eliminated because they are not concurrent with or facilitating the Will and "thou hast no right but to do thy will." This is addressed plainly in AL I:42-44:
AL I:42. So with thy all; thou hast no right but to do thy will.
AL I:43. Do that, and no other shall say nay.
AL I:44. For pure will, unassuaged of purpose, delivered from the lust of result, is every way perfect.

Crowley expands and explains these ideas in his commentary to AL I:42, "A man who is not doing his will is like a man with cancer, an independent growth in him, yet one from which he cannot get free. The idea of self-sacrifice is a moral cancer in exactly this sense...Similarly, one may say that not to do one's will is evidence of mental or moral insanity. When 'duty points one way, and inclination the other', it is proof that you are not one, but two. You have not centralized your control. This dichotomy is the beginning of conflict, which may result in a Jekyll-Hyde effect. Stevenson suggests that man may be discovered to be a 'mere polity' of many individuals. The sages knew it long since. But the name of this polity is Choronzon, mob rule, unless every individual is absolutely disciplined to serve his own, and the common, purpose without friction...It is of course better to expel or destroy an irreconcilable. 'If thine eye offend thee, cut it out.' The error in the interpretation of this doctrine has been that it has not been taken as it stands. It has been read: If thine eye offend some artificial standard of right, cut it out. The curse of society has been Procrustean morality, the ethics of the herd-men. One would have thought that a mere glance at Nature would have sufficed to disclose Her scheme of Individuality made possible by Order."

Essentially, he reinforces that that inclinations must be reworked to be most efficient vehicles for the Will - there is no arbitrary standard of right, There is no law beyond Do what thou wilt.


Reason/intellect
Also, this facet of the Eightfold Path may be taken to mean to understand thought & reason in its proper place. Reason must not intrude when in meditation, devotion, prayer, ritual, etc. because it is simply is a hindrance. Liber AL deals with the dangers of the intellect/reason very straightforwardly in AL II:27-33:
AL II:27. There is great danger in me; for who doth not understand these runes shall make a great miss. He shall fall down into the pit called Because, and there he shall perish with the dogs of Reason.
AL II:28. Now a curse upon Because and his kin!
AL II:29. May Because be accursed for ever!
AL II:30. If Will stops and cries Why, invoking Because, then Will stops & does nought.
AL II:31. If Power asks why, then is Power weakness.
AL II:32. Also reason is a lie; for there is a factor infinite & unknown; & all their words are skew-wise.
AL II:33. Enough of Because! Be he damned for a dog!

Crowley remarks in his comment to AL II:28 that "Our reasoning faculties are the toils of the Labyrinth within which we are all caught." He also says that this is a warning against establishing absolute 'standards of Right,' especially of morals & ethics, because 'each Star must go on its orbit.' Crowley continues, "To hell with 'moral Principle;' there is no such thing; that is a herd-delusion, and makes men cattle. Do not listen to the rational explanation of How Right It All Is, in the newspapers." In Freudian terms, the superego must not intrude on the Going of the Will; arbitrary standards of what is right & wrong or good & evil will almost always naturally inhibit one's Will. The Will transcends Reason and is naturally inhibited by it. Crowley talks about this in his comment to AL II:30 when he says, "There is no 'reason' why a Star should continue in its orbit. Let her rip! Every time the conscious acts, it interferes with the Subconscious, which is Hadit. It is the voice of Man, and not of a God. Any man who 'listens to reason' ceases to be a revolutionary...The Qabalists represent the mind as a complex of six elements, whereas the Will is single, the direct expression as "The Word" of the Self. The mind must inform the Understanding, which then presents a simple idea to the Will. This issues its orders accordingly for unquestioning execution. If the Will should appeal to the mind, it must confuse itself with incomplete and uncoordinated ideas. The clamour of these cries crowns Anarchy, and action becomes impossible."

Summary
*Right effort in Thelema would be the elimination of, or transforming of any thought, habit, speech, or action into something that facilitates and is efficient for the Will.
AL I:42. So with thy all; thou hast no right but to do thy will.
AL I:43. Do that, and no other shall say nay.
AL I:44. For pure will, unassuaged of purpose, delivered from the lust of result, is every way perfect.

*Also, reason/intellect is understood to be limited and dualistic. Reason & over-intellectualization inhibit the Going of the Will.
AL II:30. If Will stops and cries Why, invoking Because, then Will stops & does nought.
AL II:31. If Power asks why, then is Power weakness.
AL II:32. Also reason is a lie; for there is a factor infinite & unknown; & all their words are skew-wise.