Mirroring some other comments here, my best experiences were with overhead projectors and document cameras. My high school math teacher would sit on his desk at the front of the classroom and use an overhead projector with transparencies on a roll so he could just roll up as he wrote. In college a lot of professors used document cameras for basically the same approach. I think it was by far the most effective way to teach math because they could just write equations etc. out and also face the class, and they didn't have to deal with chalk or dry erase markers.
The soothing sound of the overhead projector, the dim light of the classroom combined with both the subject and the relaxing voice of the teacher were unbeatable for mid-day sleep during college.
