The Lectern Microphone
Now let's consider which microphone you might be using. There are four basic kinds of microphones. Although it's fine to have a preference, it's best if you're comfortable with all four types ― because when it comes time for your presentation, you may not have a choice in the matter.
The first kind of microphone is a mic which is already attached to a lectern. Although some speakers like this kind of mic because they don't have to worry about holding it, there are a couple of problems with this type of mic.
One problem is not with the mic itself, but with what its attached to ― the lectern. Although a lectern is a convenient place to put your notes, it's also a psychological barrier between you and your audience. (If you're height-challenged and the lectern is big and bulky, it might not be just a psychological barrier ― your audience may have trouble actually seeing you!)
A lectern obviously limits your movements. And although it doesn't have to limit your gestures, many speakers find that it's just so darned comfortable to hold onto the lectern ― and holding onto the lectern limits your gestures.
So I highly recommend that you not speak from behind a lectern... if you have a choice. But if you must speak from behind a lectern (and so use a lectern-attached mic), there's another problem. It's important that you remember to always speak into the microphone.
Unfortunately, it's easy not to speak into the mic without thinking about it. If you turn your head to look at your slides, or to speak to someone sitting beside you, or sometimes (depending on the sensitivity of the mic) if you even turn your head to speak to members of your audience who are not sitting directly in front of you, your voice will not be picked up as well... or maybe not at all. This causes a "drop" in your volume so that audience members may not be able to hear what you're saying. (And if your presentation is being recorded, what you say won't be recorded either!)
The fix for this is to actually position your head so that the microphone is always in front of you. In other words, if you're speaking to your right, first move your head a little to your left so that you're still speaking into the mic.
Again, this sort of gymnastics can be avoided by avoiding the lectern-attached mic, if at all possible.