Wearing a belt-Gartle during Davening:
Wears a belt throughout the day: One who is accustomed to wear a belt throughout the day is required to also wear a belt [called a Gartel] during prayer. One is, however, not required to wear a belt when he recites blessings.
Does not wear a belt throughout the day: One who is not accustomed to wear a belt throughout the day, is not required to wear a belt during Davening so long as his clothing creates a separation between his heart and below the waist. Nonetheless, the above is only for the letter of the law however there is an act of piety to wear a belt for prayer even if he is not accustomed to wearing one throughout the day. If, however, there is no separation between his heart and below his waist, such as one who is wearing a loose robe with no underclothing, then he is required to wear a belt for Davening and the saying of blessings.
Gartel wearing: It is based upon the above law that it is the custom amongst Chassidim to Daven wearing a Gartel around their waist, as although it is not worn throughout the day, and hence not technically required from the letter of the law, it at the very least contains a Midas Chassidus. Most of Jewry however is not accustomed to wear a Gartel for prayer, and various justifications have been written behind their position.
 Admur 91:2; Ketzos Hashulchan 12:5; Piskeiy Teshuvos 91:1
 M”B 91:5 in name of Magen Giborim; Ketzos Hashulchan ibid;
 See Piskeiy Teshuvos 91:1