From what I know, this is the most basic prayer of the Hebrew religion:
Hear o Israel! The LORD is our God, the LORD alone (Deut 6.4).
If you are a Jew, this is the first thing you say when you get up at sunrise. (If you're a Christian, perhaps you recite this and then, crossing yourself, you say, "Glory to the Father and to the Son and to the Holy Spirit, now and forever, amen.") This is what you say at night as you go to bed. The point is that this is a message you have got to get in your head; it's got to remain deeply embedded in your thinking about yourself and about the world. The LORD alone is your God: YHWH and no one else.
Of course, part of the reason why this has become such an important prayer is because the people of Israel, as the scriptures show us, were prone to go off in worship of other gods. This may not be so meaningful to us because we don't exactly feel the temptation to worship Baal or Molech -- at least, not in any explicit way. Truth be told, however, we tend to worship the old gods under a sanitized, secularized facade: pornography, money, sex, stuff, power, influence, and so on. These are all the old gods, though their temples are a bit differently constructed now.
I have read here and there that emphasis in the Hebrew language is accomplished through repetition. Consequently the point of repeating "the LORD" here is to single him out as the only God of Israel with emphasis. The LORD and no other; YHWH with no competitors; only and only the LORD is the God of Israel, and every other god be forgotten and dead! That is the point of the repetition.
I think it be a good practice for me to begin praying this one more often. I am not specially immune to idolatry any more than the next guy; if anything I am specially predisposed thereunto. In moments of temptation to sin of various sorts, when your eye is caught by something out of bounds, when you doubt and are confronted by struggles and travails, simply repeat: the LORD is our God, the LORD alone.