There are obviously times when you would need to take controls, but most of the time all it takes is to draw your student's attention to their mistake. For example, if they are getting slow in final, I'll just say "airspeed," and usually the student will correct it. Defensive positioning is key. I personally NEVER have my hands on my lap during takeoffs or landings, I keep my hand right there by the yoke without adding any pressure or moving it. that way I'm ready to grab it should the need arise. You definitely don't want to be adding pressure to the controls however, since they won't learn anything that way.
I agree with the above poster that it takes a couple hundred hours of dual given before you will feel comfortable, you'll get there.
I don't know what airplanes you're flying, but most training airplanes are VERY forgiving and will fly in spite of what your student does. After doing hundreds of stage checks for a 141 school where I can't help the student, I've learned to let things go pretty far before I take controls. That doesn't mean that I enjoy having students do things like try to ground loop the airplane after touchdown or stall it at 50' above the runway, but I don't take controls unless things are getting pretty bad and the student clearly isn't doing anything about it. My tolerance for that is something that developed over time as I gained more experience instructing.
Remember, training airplanes are pretty tough and can take more than you think during landings, just be careful with the nose wheel. You can't really call yourself a CFI until you've had some students try to knock fillings out of your teeth during landing.