via Lma’an Yishme’u
Who Are You Talking To? (Or, in a chassidic interpretation, “WHO” you are talking to!)
It is the custom in many shuls (synagogues) to engrave on the chazzan's shtender (podium) the words "Da lifnei Mi atah omed," Know before Whom you stand. Why do chassidishe shuls not post this reminder?
The Rebbe Rashab explained that chassidim would have this message engraved in their minds and hearts.
(ליקוטי דיבורים ע' תשצ)
The Rebbe reminded that realizing that we are speaking to HaShem is the basis of what davening is:
The basic concept of davening is to bear in mind that one stands before HaShem. This precedes other kavanos (meditations) such as pirush hamilos and surely before contemplating seder histalshelus. This is not a chassidic practice reserved for a great chossid or any chossid for that matter - this is a clear halacha in Shulchan Aruch!...
You entered shul to daven? Don't forget why you came: to daven to HaShem!
(תו"מ תשמ"ג ח"א ע' 144)
A certain unlettered chossid of the Alter Rebbe did not even know the plain meaning of much of the davening, yet he davened with intense chayus every day - Shacharis, Mincha and Maariv. It was obvious that his davening was genuine, and so earnest that it was as if he were wringing out his neshama. The other chassidim were puzzled.
When they questioned him he replied: "I only know that I heard a vort from the Rebbe on the statement, 'shamor vezachor bedibbur echad.' (The plain meaning of this phrase is that the two expressions regarding Shabbos-zachor and shamor-were both said by HaShem in the same utterance.) The Rebbe explained, "You should remember and watch over the echad, the oneness of HaShem, in every word."
Just imagine: For forty years he davened with this vort, day and night, weekdays, Shabbos, and Yom-Tov. He didn't just hear the vort: he felt it. That's a chossid.
From this story, the Rebbe drew a lesson: Every individual, regardless of his knowledge of Chassidus, can daven at length, by contemplating the Elokus (G-dliness) that enlivens every physical being.
(סה"ש תרצ"ו ע' 127, לקו"ש חי"ד ע' 224)