You need to get used to winnowing your thoughts, so that if someone says, “What are you thinking about?” you can respond at once (and truthfully) that you are thinking this or thinking that. And it would be obvious at once from your answer that your thoughts were straightforward and considerate ones — the thoughts of an unselfish person, one unconcerned with pleasure and with sensual indulgence generally, with squabbling, with slander and envy, or anything else you’d be ashamed to be caught thinking.

[ἐθιστέον ἑαυτὸν μόνα φαντάζεσθαι, περὶ ὧν εἴ τις ἄφνω ἐπανέροιτο: τί νῦν διανοῇ; μετὰ παρρησίας παραχρῆμα ἂν ἀποκρίναιο ὅτι τὸ καὶ τό: ὡς ἐξ αὐτῶν εὐθὺς δῆλα εἶναι, ὅτι πάντα ἁπλᾶ καὶ εὐμενῆ καὶ ζῴου κοινωνικοῦ καὶ ἀμελοῦντος ἡδονικῶν ἢ καθάπαξ ἀπολαυστικῶν φαντασμάτων ἢ φιλονεικίας τινὸς ἢ βασκανίας καὶ ὑποψίας ἢ ἄλλου τινός, ἐφ̓ ᾧ ἂν ἐρυθριάσειας ἐξηγούμενος, ὅτι ἐν νῷ αὐτὸ εἶχες.]

Marcus Aurelius (AD 121-180) Roman emperor (161-180), Stoic philosopher

Meditations [To Himself; Τὰ εἰς ἑαυτόν], Book 3, ch. 4 (3.4) (AD 161-180) [tr. Hays (2003)]
    (Source)

(Source (Greek)). Alternate translations:

Thou must use thyself to think only of such things, of which if a man upon a sudden should ask thee, what it is that thou art now thinking, thou mayest answer This, and That, freely and boldly, that so by thy thoughts it may presently appear that in all thee is sincere, and peaceable; as becometh one that is made for society, and regards not pleasures, nor gives way to any voluptuous imaginations at all: free from all contentiousness, envy, and suspicion, and from whatsoever else thou wouldst blush to confess thy thoughts were set upon.
[tr. Casaubon (1634)]

Let it be your way to think upon nothing, but what you could freely Discover, if the Question was put to you : So that if your Soul was thus laid open, there would nothing appear, but what was Sincere, Good-natur'd, and publick Spirited; not so much as one Libertine, or Luxurious Fancy, nothing of Litigiousness, Envy, or unreasonable Suspicion, or any thing else, which would not bear the Light, without Blushing.
[tr. Collier (1701)]

We ought, therefore, to [...] enure ourselves to think on such things, as, if we were of a sudden examined, what are we now musing upon, we could freely answer, such or such matters: so that all within might appear simple and goodnatured, such as becomes a social being, who despises pleasure, and all sensual enjoyment, and is free from emulation, envy, suspicion, or any other passion that we would blush to own we were now indulging in our minds.
[tr. Hutcheson/Moor (1742)]

Indeed you should regulate your thoughts in such a manner, that if any one should ask you, on a sudden, what is the subject of them, you may answer him without embarrassment; so that they may evidently appear to be all simplicity and benevolence, and such as become a being born for society; free from every idea of sensuality or lasciviousness; from rancour, envy, or suspicion; or from any other sentiment, which, if you were to confess it, would occasion a blush.
[tr. Graves (1792)]

A man should use himself to think of those things only about which if one should suddenly ask, What hast thou now in thy thoughts? with perfect openness thou mightest immediately answer, This or That; so that from thy words it should be plain that everything in thee is simple and benevolent, and such as befits a social animal, and one that cares not for thoughts about pleasure or sensual enjoyments at all, nor has any rivalry or envy and suspicion, or anything else for which thou wouldst blush if thou shouldst say that thou hadst it in thy mind.
[tr. Long (1862)]

[A man] ought, therefore, not to work his mind to no purpose, nor throw a superfluous link into the chain of thought; and more especially, to avoid curiosity and malice in his inquiry. Accustom yourself, therefore, tot hink up on nothing but what you could freely reveal, if the question were put to you; so that if your soul were thus laid open, there would nothing appear but what was sincere, good-natured, and public-spirited -- not so much as one voluptuous or luxurious fancy, nothing of hatred, envy, or unreasonable suspicion, nor aught else which you could not bring to the light without blushing.
[tr. Collier/Zimmern (1887)]

Limit yourself habitually to such regards, that if suddenly asked 'What is in your thoughts now?,' you could tell at once the candid and unhesitating truth -- a direct plain proof, that all your thoughts were simple and in charity, such as befit a social being, who eschews voluptuous or even self-indulgent fancies, or jealousy of any kind, or malice and suspicion, or any other mood which you would blush to own.
[tr. Rendall (1898)]

Accustom yourself so, and only so, to think, that, if any one were suddenly to ask you, “Of what are you thinking-now?” you could answer frankly and at once, “Of so and so.” Then it will plainly appear that you are all simplicity and kindliness, as befits a social being who takes little thought for enjoyment or any phantom pleasure; who spurns contentiousness, envy, or suspicion; or any passion the harbouring of which one would blush to own.
[tr. Hutcheson/Chrystal (1902)]

A man should accustom himself to think only of those things about which, if one were to ask on a sudden, What now in thy thoughts? thou couldst quite frankly answer at once, This or that; so that thine answer should immediately make manifest that all that is in thee is simple and kindly and worthy of a living being that is social and has no thought for pleasures or for the entire range of sensual images, or for any rivalry, envy, suspicion, or anything else, whereat thou wouldst blush to admit that thou hadst it in thy mind.
[tr. Haines (Loeb) (1916)]

You must habituate yourself only to thoughts about which if some one were suddenly to ask: 'What is in your mind now?', you would at once reply, quite frankly, this or that; and so from the answer it would immediately be plain that all was simplicity and kindness, the thoughts of a social being, who disregards pleasurable, or to speak more generally luxurious imaginings or rivalry of any kind, or envy and suspicion or anything else about which you would blush to put into words that you had it in your head.
[tr. Farquharson (1944)]

A man should habituate himself to such a way of thinking that if suddenly asked, "What is in your mind at this minute?" he could respond frankly and without hesitation; thus proving that all his thoughts were simple and kindly, as becomes a social being with no taste for the pleasures of sensual imaginings, jealousies, envies, suspicions, or any other sentiments that he would blush to acknowledge in himself.
[tr. Staniforth (1964)]

You must train yourself only to think such thoughts that if somebody were suddenly to ask you, "What are you thinking of?" you could reply in all honesty and without hesitation of this or that, and so make it clear at once from your reply that all within you is simple and kindly, and worthy of a social being who has no thought for pleasure, or luxury in general, or contentiousness of any kind, or envy, or suspicion, or anything else that you would blush to admit if you had it in your mind.
[tr. Hard (1997 ed.)]

Train yourself to think only those thoughts such that in answer to the sudden question ‘What is in your mind now?’ you could say with immediate frankness whatever it is, this or that: and so your answer can give direct evidence that all your thoughts are straightforward and kindly, the thoughts of a social being who has no regard for the fancies of pleasure or wider indulgence, for rivalry, malice, suspicion, or anything else that one would blush to admit was in one’s mind.
[tr. Hammond (2006)]

You should accustom yourself to think only of those things which, if someone were suddenly to ask "What are you thinking?" you could openly answer this or that, so as to reveal straightaway that everything within yourself is straightforward and well disposed, appropriate to a communal being, and without care for base pleasures or even a single one of the delights we take in our experiences, or for any rivalry, slander, suspicion, or anything else which you would blush to answer that you had in your mind.
[tr. Needleman/Piazza (2008)]

You must train yourself only to think such thoughts that if somebody were suddenly to ask you, ‘What are you thinking of?’ you could reply in all honesty and without hesitation, of this thing or that, and so make it clear at once from your reply that all within you is simple and benevolent, and worthy of a social being who has no thought for pleasure, or luxury in general, or contentiousness of any kind, or envy, or suspicion, or anything else that you would blush to admit if you had it in your mind.
[tr. Hard (2011 ed.)]

You must train yourself only to think the kind of thoughts about which, if someone suddenly asked you, "what are you thinking now?" you would at once answer frankly, "this" or "that." So, from your reply it would immediately be clear that all your thoughts are straightforward and kind and express the character of a social being who has no concern with images of pleasure, or self-indulgence in general, or any kind of rivalry, malice, or suspicion, or anything else you would blush to admit you were thinking about.
[tr. Gill (2013)]