Concentration during Shema:
One is to concentrate on every single word recited during Keiras Shema. Even if one is unable to Daven without being bothered by foreign thoughts, he is to accustom himself to recite Kerias Shema without foreign thoughts. One who does so achieves that all physical and spiritual damaging forces abstain from coming near him.
 Keser Shem Tov 1; Likkutei Yekarim 17a; Tzivas Harivash 1:18