Have you ever wondered about all those verses in the Bible that say something like, "Do this and you will live"? That is, you've come across a lot of verses like Deuteronomy 30:19-20,
"I call heaven and earth as a witness against you, that I have set before you life and death, blessing and cursing; therefore choose life, that both you and your descendants may live; that you may love the LORD your God, that you may obey his voice, and that you may cling to Him, for He is your life and the length of your days; and that you may dwell in the land which the LORD swore to your fathers, to Abraham, Isaac, and Jacob, to give them."
This particular passage was brought up in defense of free will by Erasmus when he wrote against Martin Luther. Luther's response is one in which I've used in arguments, and I find most helpful :
"It is from this passage that I derive my answer to you: that by the words of the law man is admonished and taught, not what he can do, but what he ought to do; that is, that he may know his sin, not that he may believe that he has any strength. Wherefore, my good Erasmus, as often as you confront me with the words of the law, so often shall I confront you with the words of Paul: 'By the law is knowledge of sin'—not power of will! Gather together from the big concordances all the imperative words into one chaotic heap (not the words of promise, but the words of the law and its demand)—and I shall at once declare that they always show, not what men can do, or do do, but what they should do! Even grammarians and schoolboys at street corners know that nothing more is signified by verbs in the imperative mood than what ought to be done, and that what is done or can be done should be expressed by verbs in the indicative. How is it that you theologians are twice as stupid as schoolboys, in that as soon as you get hold of a single imperative verb you infer an indicative meaning, as though the moment a thing is commanded it is done, or can be done? But there's many a slip 'twixt the cup and the lip!—and things that you commanded and that were possible enough may yet not be done, so great a gulf is there between imperative and indicative statements in the simplest everyday matters! Yet in this business of keeping the law, which is as far out of our reach as heaven is from the earth and just as impossible of attainment, you make indicatives out of imperatives with such alacrity that the moment you hear the word of command: 'do', 'keep', 'choose', you will straightway have it that it has been kept, done, chosen or fulfilled, or that these things can be done by our own strength!"
Source: Martin Luther, The Bondage of the Will (Translated by J.I. Packer & O.R. Johnston) (Grand Rapids: Fleming H. Revell, 1957), pp. 158-159.
This argument from Luther is crucial in debating the nature of the will. I realize, the Reformed would argue for three uses of the Law, but with Luther's basic point above, a myriad of arguments put forth defending free will are dismantled.