According to your profile you are very low time. As you gain experience as a CFI, you'll become much more comfortable with your abilities and will let your students make mistakes that they can learn from. Before you know it, you'll let them absolutely butcher maneuvers, approaches, and landings before taking over. Just make sure that if/when you do take over, you immediately (and calmly) tell them what happened, why it happened, and how to correct it. Then follow up with a demonstration. Don't ever get mad or visibly frustrated. I have been EXTREMELY frustrated with students before, but I didn't show it. Yelling and anger have no place in the cockpit. It's very detrimental to the learning process, and can really ruin their experience. It also scares them, because YOU are losing control in an airplane, which destroys the huge amount of trust and confidence they have in you. You will probably never get their confidence in you back if you do this. I know none of that probably applies to you, but it's so important I just wanted to throw it out there.
As far as landings, I don't ever trust anybody while wearing the CFI hat. Even if they're an advanced instrument student or 20,000 hr pilot doing a checkout (I've had both types try to kill me on several occasions!). During the base turn, I would stealthily place my feet near the rudder pedals, and put my hands on my lap. I look relaxed (for their confidence), but am ready to spring on the controls. By having your hands and feet located like this, you can spring on the controls just as fast as if you were hovering over the controls. If they feel you on the controls (feet or hands), they will have serious doubts about their abilities, so stay off them.
If it looks like it will be less than perfect, who cares? As long as it's safe. Safe means normal sink rate, not landing on the nose wheel, no porpoising, and no side loading the gear (x-winds). If they're going to bounce it, let them. It's a good learning experience for them, and VERY important for you to see if they can handle it correctly. If it's not right on the centerline, let them land and discuss how to fix it while taxiing. Also, always be very clear about what kind of landing this will be (T&G, full stop, short, soft, etc.). I always verify this and require a verbal response from them on the base turn.
I've always been a big proponent of see and do. In other words, I'll demo the maneuver or landing first, and then verbally coach them as they attempt it themselves. I absolutely hate the "follow me on the controls" method. It does not allow for the proper development of muscle memory, and usually leaves the student more confused than anything else. Bill Kershner didn't like it either, and I think he knew a thing or two about flight instructing
. Oh, and PLEASE give your students a decent preflight ground briefing on what you will be doing. Most CFIs don't do this, and it is ridiculously stupid. You cannot teach somebody about overturning tendencies or adverse yaw while they're sitting in an airplane. It just doesn't work, and makes them frustrated as hell.
Finally, there's only one area I never let my students cross into: low and slow. Either parameter (within reason) I'd let them screw up, but not both. Good luck!