Dignare Domine

A favourite short prayer of mine is the age-old versicle

V/.  Dignare, Domine, die isto [nocte ista].  

R/.  Sine peccato nos custodire.

(Deign, Lord, this day [night] to keep us without sin.)

It crops up, of course, towards the end of the Te Deum, in its third part, amongst verses taken from Holy Writ; and, before they were abolished in the 1950's, it appeared in the preces used at Prime and Compline.

More interestingly it also appears in the prayers said at several of the Hours of the Divine Office in the Byzantine Rite.  It appears at Lauds (the end of Matins) and Compline immediately after the Great Doxology, the Byzantine version of the Gloria in excelsis, heading up a set of verses from the Psalms, etc, which verses are therefore called the Kataxiôson for short; at Vespers, they are said after one of the Litanies.  

Καταξιωσον, Κυριε, εν τη ημερα [εσπερα / νυκτι] ταυτη, αναμαρτητους ϕυλαχϑηναι ημας.

(Grant, Lord, this day [evening / night] to keep us without sin.)

Greek is certainly its original language (the Latin is a very faithful translation), and it must be a very old prayer indeed.  What is noteworthy is that it is not from Scripture, although in both East and West it appears amid various sets of Scriptural quotations.

I like to say this prayer for strength to fight the good fight and keep in Gods' grace.