"Paul, how would you define "cogency" exactly"?http://triablogue.blogspot.com/2009/05/death-of-christian-apologetics.html#8876832514310712989
I would define cogency briefly as: A sound argument that is recognized by your audience to be so due to the presentation of its parts (form and content).
For example, here's a sound argument:
 God exists or December first is Christmas day.
 December first is not Christmas day.
 Therefore, God exists.
This argument is sound, not cogent.
If your apologetic argument is one that no one will recognize as sound, it's basically useless as an apologetic argument. Patting yourself on the back for merely announcing "the truth" to non-believer, and consoling yourself that "he'll be judged all the more for rejecting it", isn't doing apologetics.
Take the above argument again and let's draw another conclusion:
[1*] God does not exist or December first is Christmas day.
[2*] December first is not Christmas day.
[3*] Therefore, God does not exist.
An atheist will believe all the premises of this argument are true and so believe he's presented us with a sound argument for the non-existence of God. The problem is, like with the first argument, the atheological argument is not cogent.
However, it is the case that one of these arguments is in fact sound (as a Christian, I believe it's the first). The problem is that its soundness is hidden from us. The only way an audience would recognize the alleged soundness of the one over the other is because they already believe that God exists or that he does not.
But for both sides, there is nothing in their respective argument that rationally compels us to accept the premises.
So (one of) the apologist's job is to try to figure out how to make his arguments cogent. To get the audience to accept what you believe (know!) is true. You can do this by arguing, expanding, elaborating, for and/or on one or more of the premises.
There are many ways to do this, and a future post may be helpful in fleshing some of this out, but we can mention one thing not to do. Thwacking unbelievers on the head with your Bible and telling them that they have a reason to believe that God exists because the Bible says God exists, and the Bible is the word of God and so cannot be wrong, ergo God exists, isn't the way to go.