8. BREAKS *
The brain can only concentrate on a topic for a limited period of time. After this period, the participants are not able to learn effectively anymore. Often, this is the reason why a seminar is considered "hard". As a consequence, the participants often leave the room during sessions. This creates a hectic, noisy atmosphere.
Therefore, include regular breaks. Even five minutes of fresh air during a session can help significantly. The breaks should be planned at regular intervals during each session. Stick to the planned break times. If it is not possible to have a break at the planned time, tell the participants about it and ask them to continue for another five or ten minutes. This shows that you did not forget the break. Breaks also help to improve the quality of the group by providing room for PERSONAL COMMUNICATION.
Include many, small breaks. People can go smoke, have a little talk, go to the toilet or just go to the window and breathe fresh air. Keep the breaks short, and keep them in the schedule. Be sure that they are long enough that people have a realistic chance to go to the toilet or smoke a cigarette without returning late! During these breaks, coffee and other drinks (tea, juices) could be served. You should consider planning more breaks in the afternoon, since people are usually more tired than in the morning. Longer breaks for (second) breakfast and lunch should be included, of course.
During my time at the university, the sessions were usually 90 minutes. One professor included a 5 minute break after 45 minutes. This enabled the participants to walk around and breathe fresh air, allowing them to stay awake for the second half of the chemistry lessons.
In his book Accelerated Learning [CR], Colin Rose suggests taking a ten minute break after 50 minutes of instruction.