Hashem Elokeichem Emet / El Melech Ne'eman
The shema is comprised of 245 words. There is a tradition which strongly encourages one to ensure that the words of the shema total 248, the number of limbs in a person's body. In order to accomplish this, the one leading the services repeats the last three words of shema, "Hashem Elokeichem Emet", thereby bringing the total number of words to 248 through this three-word recitation. Common custom is to consider hearing the leader's repetition of these words as if one had done so oneself. One who wishes to repeat these words as well is entitled to do so, though this is not customarily done.
One who is praying alone does not have the advantage of hearing someone recite these three words. As such, there are a number of options for one who is praying alone to still benefit from a 248 word recitation of shema. One of the options offered is for one to concentrate extra carefully on the first letter of each of the 15 words which follow the shema of shacharit. This method numerically corresponds to God's name three times.Another more widespread practice is to recite the three words "El Melech Ne'eman" before reciting the shema at both shacharit and ma'ariv. Some always recite El Melech Ne'eman even when only reciting the first paragraph of shema, such as before going to sleep at night. One who is praying with a minyan should not say this three word formula.
Nevertheless, those who follow the customs of the Arizal are not to recite "El Melech Ne'eman" before the shema, at any time. This is out of a concern that doing so is an unnecessary, and therefore prohibited, interruption between the blessing that precedes the shema and the shema recitation itself. So too, according to this school of thought, one is to fill in the missing three words of shema at the end of its recitation, not at the beginning. As such, followers of the Arizal repeat the last three words of shema -"Hashem Elokeichem Emet"- in order to arrive at 248 words.
There also exists a custom for the leader to repeat the words "Ani Hashem Elokeichem Emet", due to the concern that the word "Emet" truly belongs to the prayer following the shema, and not to the shema itself. Should this be true, one would have not discharged the requirement to have 248 words. This custom, however, is hardly to be found. One who is still in the midst of kriat shema when the leader appears ready to recite "Hashem Elokeichem Emet" should pause and listen to the leader, which will count towards one's personal 248 word recitation of kriat shema.
 Kaf Hachaim 61:10
 O.C. 61:3
 Kaf Hachaim 61:13
 Rema O.C. 61:3, Kaf Hachaim 61:14
 O.C. 61:3. See Rema for additional interpretations
 Rema O.C. 61:3
 V'shav V'rapeh 2:7
 Rema O.C. 61:3
 Kaf Hachaim 61:15,19
 Beit Yosef 61, Kaf Hachaim 61:19
 Beit Yosef O.C. 61
 Piskei Teshuvot 61:3