Gemara

13b (Beraisa - R. Meir): "Shema Yisrael Hash-m Elokeinu Hash-m Echad" -- intent is required until here.

(Rava): The Halachah follows R. Meir.

61b (Beraisa - R. Eliezer) Question: Why must it say both "uv'Chol Nafshecha" and "uv'Chol Me'odecha"?

Answer: [Even] if one's life is dearer to him than his money, he must love Hash-m with all his life. [Even] if his money is dearer to him than his life, he must love Hash-m with all his money.

R. Akiva says, "uv'Chol Nafshecha" teaches that one must love Hash-m even when He takes your life.

When R. Akiva was taken to be executed, it was the time for Keri'as Shema. They were tearing off his skin with iron combs, and he was accepting upon himself Ol Malchus Shamayim.

His Talmidim: Rebbi, to this extent?! (You are able to do so even amidst such pain?!)

R. Akiva: My entire life I was troubled by the verse "uv'Chol Nafshecha," even when He takes your life. When will I be able to fulfill it? Now that I have the opportunity, will I not fulfill it?!

Rishonim

Rashba (Teshuvah 5:55): Every time one reads Keri'as Shema, he must absolutely resolve to give up all his lusts and desires, and his soul, from limbs up to forfeiting his life, and his property, for the sake of Yichud Kedushah of Hash-m's name, and acceptance of His kingship. One should picture a situation that demands Mesiras Nefesh, and that he fulfilled. Through this, it is as if he was killed for Hash-m. Whoever resolves is credited as if he fulfilled. "Al Levavecha" means that these matters must be settled and engraved in his heart. A real acceptance is not through fear of punishment, and he would prefer not to be commanded. A full Avodah is when he is happy that he is commanded.

Sefer Chasidim (222): Some are killed for Kidush Hash-m. Others die normally on their beds, but had they lived at the time of decrees against the Mitzvos, they would have died for Kidush Hash-m. Why should they lose (the reward for this)? Indeed, (they receive the reward,) just is deducted the benefit they received that they did not die, and were able to live and have children. Those who were killed receive reward corresponding to life in this world that they forfeited. One Yisrael saw in a dream someone who was killed for Kidush Hash-m. He said that all those who resolved in their hearts to die for Kidush Hash-m have a share with him in Gan Eden.

Tur (OC 61): A Medrash says "when a mortal king sends his letter to the Medinah, everyone stands, uncovers his head, and reads it with awe, fear, trembling and sweat. However, Hash-m says read my letter, i.e. Keri'as Shema. I do not exert you to stand or uncover your heads. You may read it while going onr." However, awe, fear, trembling and sweat are required.

Poskim

Shulchan Aruch (OC 61:1): One reads Keri'as Shema with awe, fear, trembling and sweat.

Bach (DH v'Hevi): The Tur brought Rav Amram Gaon who says that this applies to whenever one reads Shema, i.e. during the week and on Shabbos and Yom Tov. Even though the latter are days of Simchah, one must read it with awe and fear. I.e. when he reads it, he intends to accept Ol Malchus Shomayim, to be killed for Kidush Hash-m. This is b'Chol Nafshecha. Then, he reads it with awe, fear, trembling and sweat.

Taz (1): The Medrash connotes that one may read Keri'as Shema with an uncovered head. Shabbos 118b forbids this, and the Tur (Siman 2) calls this lightheadedness! I answer that it is lightheaded because it looks like there is no yoke on him. Here, one must read with awe, trembling and sweat. Even if his head is uncovered, he shows awe! However, nowadays it is a Nochri ritual (to uncover the head in their prayer houses), so it is forbidden due to following Nochri rituals.

Gra (DH Yikra): We say "until here requires Kavanah" regarding b'Di'eved. L'Chatchilah, one must have Kavanah in all of Keri'as Shema.

Ateres Zekenim: One must also intend to be Yotzei the Mitzvah of Keri'as Shema.

Chayei Adam (21:11): There is no time to picture Mesiras Nefesh while saying Echad. Rather, one intends after saying it.

Orchos Rabbeinu (the Steipler, 1 p. 53) and Pninei Tefilah (p.24, citing ha'Gaon R. Y.S. Elyashiv Ztz"l): One may finish the intent afterwards if one began to intend during Echad.

Ma'aseh Ish (the Chazon Ish, 1 p.129): Retroactive intent helps if he did not yet begin Baruch Shem.

Aruch ha'Shulchan (1): While reciting Shema one should picture that even if he is tortured greatly it will be nothing in his eyes, due to his love of Hash-m, like R. Akiva, whose skin was being torn with iron combs and he recited Shema. Many Kedoshei Elyon recited Shema while they were killed and burned.

Tzelach (Berachos 61b DH Omru): R. Akiva had prayed to die while saying Shema, in order to fulfill "uv'Chol Nafshecha". His Talmidim thought that he held that Kidush Hash-m is only until death. They said that one must endure all torture, and advised him to do so to increase his reward.

Alei Shor (2, p.362): A sincere acceptance includes Mesiras Nefesh in life for Torah and Mitzvos, and to use every limb only for His service: to look only at what is proper, not to hear forbidden talk, not to speak idle or forbidden words, or eat purely for the sake of Ta'avah...

Mishnah Berurah (2): Eliyahu Rabah brings from the Yerushalmi (Berachos 1:5) that Keri'as Shema alludes to all Aseres ha'Dibros. "Hash-m Elokeinu" alludes to Anochi Hash-m Elokecha. Hash-m Echad alludes to Lo Yihyeh Lecha (the Isur of idolatry).

Note: Idolatry is explicit in Shema - "Hishamru... va'Avadtem Elohim Acherim"! Perhaps Chachamim wanted people to think about (and accept) the Isur of idolatry while accepting the yoke of Shamayim in the first verse. Perhaps Toldos Yitzchak on the Yerushalmi alludes to this.

Mishnah Berurah (2): V'Ahavta alludes to Lo Sisa, for one who loves the King will not swear falsely in His name. Lo Sachmod is alluded to in "Beiesecha", and not (to desire) another's house. V'Asafta Deganecha (and not your friend's grain) alludes to Lo Signov. V'Avadtem Meherah alludes to Lo Sirtzach, for one who kills will be killed. Lema'an Yirbu Yemeichem alludes to Kivud Av v'Em (for which the Torah promises long life). V'Lo Sasuru... v'Acharei Einiechem alludes to Lo Sin'af. Lema'an Tizkeru alludes to Zachor Es Yom ha'Shabbos, for Shabbos is equal to the entire Torah. Ani Hash-m Elokeichem alludes to Lo Sa'aneh b'Re'echa Ed Shaker. One must contemplate these when reciting Shema, so he will not come to transgress any of them.

Note: Even though Sanhedrin 86a derives that Lo Signov in the 10 utterances refers to kidnapping, v'Asafta Deganecha hints to the Isur of Geneivah, and "Geneivah" applies to stealing money or people.

Mishnah Berurah (4): The Prishah says that Keri'as Shema is compared to the king's command, to teach that one should not read it quickly and jumble the words. Rather, he reads it slowly, word by word, and pauses between matters, like one who reads the king's command. He reads very slowly, each command by itself to understand it properly. So one reads Keri'as Shema, to understand every command and punishment of the Great King.

Kaf ha'Chayim (4): The Avudraham says that the Ayin of Shema and the Dalet of Echad are big, to allude to Edus (testimony that Hash-m is One).

Kaf ha'Chayim (5): Just like one who reads Shema incriminates himself (Berachos 14b), the same applies to one who reads it without Kavanah (Sefer Charedim and others).

Mishnah Brurah (25:14): One must put love of Hash-m in his heart when saying "v'Ahavta". If not, he incriminates himself.

Kaf ha'Chayim (6): One should sweat a lot if he realizes how many times he read Shema without the proper fear, and there will be a judgment for this! Habit becomes nature.

Mishnah Berurah (5:3): When saying the first verse, one must say Hash-m's name with Kavanah for how we pronounce it (Master of all) and the way it is written (He was, is and will be).

Shulchan Aruch (6): When saying the 'Ches', he coronates Hash-m over (the seven levels of) Shamayim and Aretz. When saying the Dalet, he thinks that Hash-m is alone in His world and rules over the four directions.

Kaf ha'Chayim (28): He also accepts the four death penalties. This atones for his sins.

Kaf ha'Chayim (24): When saying the Aleph, he thinks that Hash-m is one.

Shulchan Aruch (63:4): The primary Kavanah in Keri'as Shema is in the first verse. Therefore if one said the verse without intent, he was not Yotzei, and he must repeat it.

Mishnah Berurah (12): Also Baruch Shem Kevod... must be repeated if it was said without intent.

Mishnah Berurah (13): This is even if he intended beforehand for the Mitzvah.

Mishnah Brurah (14): He must repeat the entire paragraph from the beginning. If he remembered in the middle of another Parshah, he finishes that Parshah, and then repeats Parshas Shema.

Bi'ur Halachah (101:1 DH ha'Mispalel): Even if he intends while saying the words, one must be very careful that his mind not be distracted in the middle. If it was, perhaps he was not Yotzei.

Igros Moshe (5:5) and Halichos Shlomo 5(32): B'Di'eved, he was Yotzei.