Having taught school and later, as a trial lawyer, I learned to use my voice much more strongly than in normal conversation and to make eye contact with those listening. Using a weak voice will cause people to lose interest. Move around a bit while talking unless the room is big enough that it is necessary to use a microphone. Even then make eye contact. As for guest speakers the audience needs to be set straight by the program chair. Ours once said that unless one was on the waiting list for a life saving organ transplant, everyone should take out their cell phones and turn them off. Unless one is a CIA agent or a brain surgeon with a patient recently operated on what possible excuse would one have to leave that infernal cell phone on.
Jane