There is another type of preparation that a person needs to do in a anticipation of a mitzvah — preparing one’s body, as it is written, Prepare towards your God, O Israel (Amos 4:12). Examples of this are washing one’s face before the morning prayer or preparing oneself with the clothing they will wear when praying. Even if it should happen that one is praying at home alone, he should see to it do dress in the manner that people appear before high ranking individuals. Similarly, when one is engaged in Torah study, in which case the Shechinah is present, one should dress in a dignified manner and sit in a dignified manner the way one would sit in front of a king. Likewise, when one is reclining at his table to eat, and especially when one is reciting Bircas Hamazon (Grace After Meals) to the King of Glory, one must sit in a dignified manner, for it is the equivalent of the Table that is before Hashem. There is a well-known story that happened with our master, Rabbi Yisrael Najara, in the time of the Arizal. [One holy Shabbos eve, Rabbi Yisrael Najara was singing at his table, as was his good custom, and the Arizal of blessed memory saw thousands and myriads of angels arising and descending to Rabbi Yisrael’s house to listen to the praises he was singing, for all the liturgy he wrote was composed with Divine Inspiration. But then the master, Arizal, saw that another angel appeared suddenly and drove away all the other angels because Rabbi Yisrael was sitting at his table that was before Hashem with his arms uncovered and also because he was not wearing a hat. When the Arizal sensed this, he sent two of his disciples to Rabbi Yisrael to reveal to him that there had been thousands of God’s angels who were delighting in his songs and praises but because he was sitting at his table in a manner that lacked dignity, they departed to on High. When the sage heard this, he was seized with trembling and shaking knees; he shuddered and stood, still shaking. He then sat once again at his table — this time with great dignity — and resumed his singing which delighted God and mortals. The angels instantly returned as they had appeared in the beginning. They were overjoyed and happy, celebrating with great delight, like people who gather and come to see the merrymaking performed before a groom and bride. The master, the Arizal and his disciple watched all this in wonder.]
How good it is to be meticulous and don an outer garment during prayer time.