Having been a student in a four hour class prior to teaching I found the following helped:
a. breaks 5-10 minutes every hour or 1.5 hours
b. various teaching methods: lecture, group activities, hands on, review questions and discussion, etc. I agree guest lecturers are great.
The classes can be tough especially in the evening after a full day of work for students and instructors - some days will be better than others - try your best!
