Give them some food. If it is four o clock in the afternoon, your audience is tired and their blood sugar is also probably low. Stop at the store. Buy some goodies. Fruit is excellent.
Candies are inexpensive and give people something to suck on.
Small cookies or nuts let people munch away. Ask for their input. If it s a small group, ask each person what he or she wants from the session. People have an opportunity to tell you what will keep them motivated. If you are not going to address any of the subjects and issues your audience brings up, then you are going to have an audience interest problem.