Chazal (Shabbos 10b) learn from this Pasuk that when one davens [to Hashem] he/she must act as if they are standing in front of a king and talking to him with trepidation and awe.

Therefore, it is extremely important when davening [or making any Brachos] to be dressed appropriately and respectfully. During davening one should be clothed in garments that cover his/her body properly. One should preferably not daven while wearing short pants or other articles of clothing with which he would never stand before a king or other dignitary.

in extreme situations (Sh’as HaDchak) when covering up adequately is impossible, as long as the Makom HaErvah (areas of nakedness) is covered one may say Krias Shma, Brachos and all of davening except Shemona Esrei which may not be said in any state of undress, even if by doing so you will miss the Z’man Tefilah (the proper time to say Shemona Esrei).

However, if one said Shemona Esrei while in a state of undress, as long as the Makom HaErvah was covered he/she has satisfied their obligation, and needs not daven again.

2) One should preferably wear an outer garment (Jacket or robe) while davening Shemona Esrei. Rav Chaim Kanievsky Shlita is of the opinion that a Jacket should be worn even for the other parts of davening, not just Shemona Esrei.

Even by Shacharis when one is wearing a Tallis, a Jacket should still be worn.

Preferably, one should not daven while wearing pajamas and should put on a robe on top of his pajamas for davening. (this is very common when one is sick at home, and isnt planning to get dressed that day.)