Debate has a long history in humankind, dating back at least as far as the times of Plato and Aristotle. In more recent times, historians give credit to a debate for helping a president win the election -- when John F. Kennedy's tan complexion and handsome looks in the first televised U.S. presidential debate helped him curry favour with the viewing audience over a runny-nosed and unshaven Richard Nixon. Take a cue from past presidents when you greet the audience in a debate, and do it the way they did on their path to the White House.
Wait for the moderator to indicate that it's your turn to speak. You will already know, before the debate begins, whether you or your opponent speaks first.
Greet the audience as soon as it's your turn.
Greet the audience as members and guests of the organisation that sponsors the debate. In October 2008, President Obama said, "I want to first, obviously, thank Belmont University, Tom (Brokaw, the moderator), thank you, and to all of you who are participating tonight and those of you who sent e-mail questions in."
Finish the greeting by moving immediately into your response to the moderator's question.
It's not mandatory that you greet the audience. Neither Kennedy nor Nixon did during their historical debate.