In order to prepare for prayer one must think about God’s greatness and the kindnesses He does for the Jewish people in general and each individual in particular. He should also realize that the ability to stand before God in prayer is itself one of the kindnesses He does for us, for man by himself is not worthy of it.

How is the heart prepared? Before starting to pray one should sit silently for a while without doing anything. He should remove all other thoughts from his mind and think about the greatness of the One who spoke and the world came into being: His wonders, His awesome and mighty acts, and the kindness He did for His nation. One should also think about the good and kind things that God did for him…. He should think of his own deficiency and lowliness vis-à-vis his Creator…. He should think that he wants to now get up before the King, and to supplicate and request of him his requests and desires. The person is, in truth, unfit and unworthy to stand before Him, if not for the great kindness and perfection of the Creator who permitted and commanded him to pray, even though his service may fall short [of what it should be]. Then he should get up and pray. (Rabbi Menaĥem di Lonzano, Derekh Ĥayyim, p. 49)

Further reading: For more on proper preparation for prayer, see A Concise Guide to the Sages, p. 457.