Does Halacha require or encourage a person to wear a belt during prayer?
The Gemara in Masechet Shabbat (10) addresses the obligation to make oneself look presentable in preparation for prayer. The Sages inferred this requirement from the verse in the Book of Amos (4:12), "Hikon Likrat Elokecha Yisrael" "Prepare to greet your God, O Israel!" Among the preparations the Gemara specifies in the context of this Halacha is girding oneself with a belt. Accordingly, the Shulhan Aruch rules (Orah Haim 91:2; listen to audio recording for precise citation) that one must make a point of wearing a belt for Tefila. He adds that this applies even if a person already has something around his waist such as undergarments or pants that separates between his heart and the lower part of the body. Such a separation does not suffice for prayer, which also requires wearing a belt on top of one's clothing.
The Mishna Berura (commentary to the Shulhan Aruch by Rabbi Yisrael Kagan, 1839-1933) clarifies that this Halacha applies only to somebody who normally wears a belt as part of his usual garb. Such a person must wear a belt for prayers because it would be inappropriate to appear before God, so-to-speak, with a lower standard of dress than his usual attire. If he always goes about with a belt, then he must adhere to this standard when approaching God in prayer.
If, however, a person does not normally make a point of wearing a belt, then he is not, strictly speaking, required to wear a belt for prayer. Since he maintains his usual level of dress, he may pray even without a belt. Nevertheless, the Mishna Berura advises as a "Midat Hasidut" a measure of piety to ensure to wear a belt during prayer even if one normally goes about without wearing a belt.
Summary: A person who normally wears a belt as part of his standard attire must wear a belt during prayer. Somebody who does not normally wear a belt is not, strictly speaking, required to wear a belt for prayer, though doing so constitutes a "Midat Hasidut" a special level of piety.
