In many ways what's happening on your face when you're presenting trumps everything else: the words you speak; the way you stand and move; what you do with your hands. Your face is the primary focus for the audience [or should be unless you have too many slides] and it gives them clues about your confidence, your enthusiasm and your competence.
Most people are less aware of their facial expressions than they are of other aspects of their body language. You frequently don't realize when you're frowning or grimacing or looking bored or annoyed. This may be because you've never seen yourself giving a presentation and so haven't noticed the disconnect between what your face and mouth are saying. Or it may be because you actually feel threatened or bored or angry but you think those feelings are hidden and not reflected on your face. In either case the audience will take your expression at face value [pardon the pun] -- if you look bored, they will assume you are.
Managing your facial expressions requires that you be aware of them. So the first step is to videotape yourself in several different settings -- at a meeting, giving a presentation, listening to a presentation -- or have a friend or colleague observe you. Then assess what's happening on your face. Are your expressions appropriate? Do you look engaged? Does your face say "no" when your voice is saying "yes"? As you prepare for your next presentation, work on putting some intentional focus on what your face is communicating. Videotape yourself again to detect progress.
One of the most oft-metioned components of facial expressions is eye contact. Looking audience members directly in the eye allows you to connect with them on a human level. Strong eye contact communicates sincerity and confidence and shows that you are engaged in what you're saying. It also makes it more likely that your overall facial expression will be appropriate. The goal is to meet an audience member's eyes for 3-5 seconds and then move on to the next person. Try not to "sweep" across the audience, don't favor one section of the audience over another and keep your pattern of eye contact random so you're not continually going from right to center to left and back again.
Perhaps the most powerful aspect of facial expressions is the smile. Unfortunately it's not seen that often in business presentations -- perhaps for fear of looking unprofessional?? -- but it is a stunning way of connecting with your audience. Obviously you won't want to grin through your entire presentation, but a sincere smile, when appropriate, will communicate your enthusiasm for your topic and will help make your audience comfortable.
Bottom line....your face can speak volumes. Make sure it's complementing what's coming out of your mouth.
More Reading on Body Language:
flickr/The Faces C.C. 2.0