Microphones can be a real problem. Very few of us use them frequently and so, when we have to talk into them in an already nervous state, we can easily make elementary mistakes.
As a general rule, try to speak more clearly when you are using a microphone. (It will probably help if you speak a bit more slowly.)
If you have a free-standing mike, step back from it a bit. This will enable you to speak louder and to vary your tone and inflection. If you are too close, your voice will sound monotonous and your audience will fall asleep.
Don’t turn your head away from the microphone while you are speaking. But do turn it away if you cough or sneeze!
Any little movement you make, such as shuffling your papers, will be amplified by the mike. Cut out the nervous gestures!
If you are wearing a clip-on mike, make sure it is not rubbing up against some clothing or jewellery. The noise this makes could ruin your presentation.
If you have a radio-mike, make sure it is switched on when you are presenting and switched off at all other times. This particularly applies when you go to the restroom!