Too many variables.
Will your class be in a large room or a small room? Room size, I've found, greatly influences students' abilities to concentrate. They're much better able to listen closely and write down what you have to say in a large room where there's some space between each student.
In a small room, the heat builds up, the smells build up. It's like being on a crowded elevator. They get nervous and antsy. I'll shop at a quiet grocery store over the lower priced crowded one, just because crowds get to me.
Then the personality of each class is different, and the students themselves change from year to year. What worked in the past no longer works with the new crop.
You have to change to best reach the new kind of student you have in front of you. Today, generally speaking, the student has a shorter attention span, due to the multitasking on electric devices like cell phones and Ipods. They are often people who are used to teachers teaching to the standardized high school exams they have to take (TASP, TAKS). They like to be told exactly what they have to do to get the grade. They like to be given samples of past work, which is a bit hard because you want to respect the privacy of past students.
Teaching is therefore an adventure. I like to be in control, but you're not completely in control in the classroom. Each time you walk into a class room, there can be surprises. A certain amount of improvisation is called for. What do you do with the one student who is totally different from everyone else is the classroom?
It's a bit like canoeing down a river you've never been down before. You don't know what rapids are up ahead. You just run them the best you can. I even get, in college, the trickster who asks a series of stupid, rapid fire questions to slow the advancement of the class. An old high school trick. I usually just call them on it right off. I laugh with them at what they're doing. Everyone knows the game they're playing, and many in college in the class don't appreciate it.
Teaching, it's an adventure each class. That's what keeps me interested and coming back. Every day I get surprised. Something that once worked beautifully now flops. Something new is a big success. A lot of times, the new class I'm teaching is the best class I teach on that subject. The material is fresh for me and I'm closer to their level. We're on a journey of discovery together.