I'm not gonna tell you NOT to think. That would be lunacy. But I suppose the opposite trap to fall into would be to overthink.

If you get flustered, fall back on that occlumency exercise I taught: count your breaths.

(Huh, the ironic thing is, if he notices what you're doing he'd approve of it. And he'd tell you it's a Noble Arts exercise. I ran across it in the textbook.)