I appreciate the reinforcement that attention spans last about 10-15 minutes. In my personal experience, it's less. I try to follow an 8 minute rule (not supported by lit, but shared by early mentors). After 8 minutes of talking, do something else or you'll loose your audience. 

I found this list particularly helpful:

At the end of their first class meeting students should have:

A sense of where they’re going and how they’ll get there.
A feeling that the other members of the class are not strangers.
An awareness that you care about their learning.
An expectation that the course will be both valuable and enjoyable.

"Proper Prior Planning Prevents Piss Poor Performance" (attributed to military planning)

Also, being human and personable is just as important at being knowledgeable. 

Most learning takes place outside of the classroom. The classroom, however, sets to tone and cadence for teaching, followed by learning. By being prepared and professional, I create the environment where learning is optimized.