Lifting Foreign Thoughts
Foreign thoughts during Torah and prayer are fallen letters that come to be raised to their root.
The Meor Einayim begins with a question: how can a limited person attach to Hashem, who is without limit? Hashem gave us the Torah and, as it were, contracted Himself into it, so that a person who clings to its letters and to the words of prayer clings to Him. Yet foreign thoughts come and disturb us just then. The Rebbe answers that no thought exists without letters, so a foreign thought is a set of letters that fell because of a person's own deeds. It arrives when he tries to attach to Hashem so that he may lift it up, not in order to confuse him. He should raise those letters to their root, with love and awe, and he does it by putting all his strength and life into the letters of his learning and prayer.
Source: Meor Einayim, Nasso, section 1 (on Bamidbar 4:21-22)
The next time a stray thought breaks into your davening or learning, do not fight it and do not scold yourself. Recognise that it came to be lifted. Ask what it is made of, a worry, a wish or a fear, and turn it into one sentence to Hashem. Then return to the word you were saying, and say it with all your strength, as if that word were the whole prayer. Try this three times this week and see whether the distraction loses its grip.