Quoted from John Donne's sermon, preached in 1640 at the Funeral of Sir William Cokayne, Knight and Alderman of London (from "Sermons", #80):
When we consider with a religious seriousnesse the manifold weaknesses of the strongest devotions in time of Prayer, it is a sad consideration. I throw my selfe downe in my Chamber, and I call in, and invite God, and his Angels thither, and when they are there, I neglect God and his Angels, for the noise of a Flie, for the ratling of a Coach, for the whining of a doore, I talke on, in the same posture of praying; Eyes lifted up; knees bowed downe; as though I prayed to God; and, if God, or his Angels should aske me, when I thought last of God in that prayer, I cannot tell: Sometimes I finde that I had forgot what I was about, but when I began to forget it, I cannot tell. A memory of yesterdays pleasures, a feare of to morrows dangers. A straw under my knee, a noise in mine eare, a light in mine eye,. an any thing, a nothing, a fancy, a Chimera in my braine, troubles me in my prayer. So certainly is there nothing, nothing in spirituall things, perfect in this world.
Cf. the following Yerushalmi:
R. Hiyya said, “I never concentrated during prayer in all my days! Once I wanted to concentrate, but I thought about who will meet the king first: the Arkafta [a Persian high official] or the Exilarch [the head of the Jewish community in Persia]?”Shemuel said, “I count clouds [or “flocks of birds”] [during prayer].”Rabbi Bun bar Hiyya said, “I count the layers of stones in the wall [while I pray].”Rabbi Matnaya said, “I am grateful to my head, because it bows by itself when I read ‘Modim’!”[Yerushalmi Berakhot, end of 2:4 ]
(N.B. R' Chaim Kanievsky's explanation (one of those question and answer books) is that their minds were so concentrated on their learning that they had trouble focusing during davening.)