Is it proper to stand while reciting Shema?
The Talmud in Masechet Berachot 10B records a debate between Beit Hillel (the school of the scholar Hillel) and Beit Shammai (the school of the scholar Shammai) concerning the Torah's requirement that one recite Shema "Be'shochbecha U've'kumecha" ("as you lie down and as you rise"). Beit Hillel understood this verse as a reference to the proper time for this recitation: one must read the Shema in the evening, when people normally lie down, and in the morning, when people generally rise from their beds. Beit Shammai, by contrast, interpreted this phrase as establishing the required position during the recitation of Shema: in the evening, one must lie down while reciting Shema, and in the morning, one must recite Shema while standing. It thus emerges that according to Beit Hillel, the Torah imposes no requirements concerning the proper position while reciting Shema, whereas Beit Shammai requires standing for the daytime Shema and reclining for the nighttime Shema.
Halacha, of course, follows the view of Beit Hillel, that one fulfills his obligation regardless of the position in which he recites Shema (Shulchan Aruch 63:1). The Shulchan Aruch (63:2) writes that a person who makes a point to follow Beit Shammai's stringency, and rises from his seat for the recitation of the morning Shema, is considered an "Avaryan" – a sinner – for opposing the accepting view of Beit Hillel. The Mishna Berura (commentary to the Shulchan Aruch by Rabbi Yisrael Meir Kagan, the "Chafetz Chayim," Lithuania, 1835-1933) (ibid Seif kattan 5) extends this rule to a case of one who stands not for the purpose of satisfying Beit Shammai's position, but rather to enhance his concentration. Even though he does not stand with the intention of following Beit Shammai's view, such a person acts improperly by appearing to reject the accepted position of Beit Hillel. Nevertheless, as the Mishna Berura cites from the Peri Megadim (Halachic work by Rabbi Yosef Teomim, Germany, 1727-1792), if one did stand from his seat for the recitation of Shema, although he has acted improperly, he has nevertheless fulfilled his obligation and need not repeat the Shema. See Halacha Berura, Helek 4, page 361 that says in such a case, it is preferable to repeat the Sheam after the Tefilah.
Summary: It is preferable to sit while reciting Shema, and one should certainly not rise from his seat specifically for the Shema recitation, even if he feels that this would enhance his concentration. Nevertheless, if one did stand during the recitation of Shema, he has fulfilled his obligation and is not required to repeat the Shema, but he should preferably repeat the Shema after the Tefilah.
