By Rabbi Chaim Chazzan
What intention is one supposed to have while reciting Shema?
Before one recites Shema one should have the intention that he is about to fulfill the mitzvas asei of Krias Shema.
One should preferably concentrate while reciting all three passages of Krias Shema. However, the minimum requirement is to concentrate during the recital of the first possuk of Shema and “Boruch Shem,” because they express one’s acceptance of Hashem’s Kingship. If a person recited Shema and did not concentrate for the first possuk or “Boruch Shem,” he must go back and recite them again.
The minimum intention required in order to fulfill the obligation is to understand the meaning of what he is saying, i.e. for “Shema” that he accepts that Hashem is our G-d and is One without any ‘partners’, and for “Boruch Shem” that His Sovereignty is forever.
As with the rest of davening, one should have in mind the specific kavanos for Hashem’s names: For Adnai that He is master of the world that was, is and will be, and for Elokeinu that He is strong and mighty and that He is omnipotent in both the Heavenly and the terrestrial realms.
Chazal say: Whoever prolongs his enunciation of the Daled in the word echod will have his days and years prolonged. The Ches should also be prolonged somewhat, so that one can mentally affirm the sovereignty of Hashem over the heaven and earth (the gematria of Ches alludes to the seven heavens and the earth). The Daled should be prolonged more, so that one can contemplate on how Hashem is alone in His world and rules over all four directions of the world.
It should be noted that the actual enunciation of the Daled should not be drawn out, for whenever the final letter of a word is not vocalized by a vowel mark below it, its enunciation should be cut short. Rather, one should think at length about affirming Hashem’s sovereignty in the four directions while enunciating the Daled and afterwards, before proceeding with “Boruch Shem.“
Reb Elimelch of Lizensk writes that one should think of how he is prepared to sacrifice his life for Hashem’s unity and he should imagine in his mind a scenario of a goy actually torturing him for his refusal to deny Hashem’s unity.
 שוע”ר סי’ ס ס”ה
 שוע”ר סי’ סג ס”ה
 שם. וראה אג”מ בהע’ הבא
 לכאו’ כוונות אלו אינם לעיכובא אפי’ בפסוק ראשון דק”ש, וכ”פ ובאג”מ או”ח ח”ה סי’ ה. אבל בעל הא”א מבוטשאטש בסידור תפילה לדוד ניסח תנאי שיאמר לפני התפילה שכל פעם שמזכיר השמות כוונתו לפי הכוונות, וכותב שתנאי זה אינו מועיל לפסוק ראשון דק”ש
 שוע”ר סי’ ה ס”א וג’
 ברכות יג ע”ב
 שוע”ר סי’ סא ס”ו וז’
 צעטל קטן אות ב.