A lot of it has to do (in my experience) with how they perceive your attitude. If you seem like you're in constant fear for your life and ready to snatch the aircraft from certain destruction at every moment, they'll become demoralized and wind up freezing up/getting tunnel vision. Practice looking unconcerned even when every nerve in your limbic system is screaming "we're gonna die!". Keep your voice calm and controlled until it's time that you have to take over. When and if you do have to take over, laugh it off. "Ok, see, actually, that was fine until we almost hit the runway with the nose down. Your power control was good, you just forgot the pitch part of the equation. You turned base on speed and in the right place, you just sort of stopped slowing down at that point and came in hot on final with the nose too low." (Or whatever the case may be). Praise whenever possible, and work your criticisms in between the praise. Remember what it was like to be over in the left seat with no idea what the hell was going on. Remember how easy it was to get demoralized by the tiniest slipup and how totally overwhelming it was to start out.
Every time you take the controls, you're reinforcing the student's natural belief that flying is insane and they're going to kill themselves trying to do it, so do it as little as you possibly can.
Also, think about what you're trying to learn to do in the same way that you think about what they're trying to learn to do. Lots and lots of people have learned to be "hands-off" CFIs. No one (no one I know anyway) started out as one. Keep doing it, remember what you've learned, and you'll get it. Just like them!
PS. Practice yawning. Seriously. Yawn when you don't need to. Yawn constantly. Stare out the window when you're in cruise, like you're on a long car ride and you're getting ready to start singing "100 bottles of beer". Adopt the tone of voice you might have with a buddy when you're talking about beer or women. You get the idea.