I remember a track instruction day where I asked the racer who was coaching us on tips on how to drag knee.
His response was, "Why would you want to do that? I'm trying so hard to get my knee out of the way!"
And now I'm finding that knee-dragging is actually making me slow. As soon as I touch down, I stop leaning, and my lean angle and corner speed sucks because of it. Now I have to focus on keeping the knee closer to the bike - to get my knee out of the way.
Just work on your corner speed, the knee will get closer to the ground. And when that happens, you'll have to figure out a way to get your knee out of the way as well.