If there's a microphone on hand, then use it-or at least appear to use it. People think they can hear better if they see a speaker standing in front of a microphone.
Wherever you are going to speak, enquire beforehand if a microphone is to be provided and if it is then test it in advance. It won't enhance the start of your speech if you have to open with the words ' Testing... Testing... One, two, three, four...
Some microphones have on/off switches. Make sure yours is on before you launch into your speech, but don't have it on too soon.