1. Look and dress the part. Get ready for a Web conference session the same way you prepare to meet or teach in face-to-face settings: after all, with the camera, you are face-to-face. Wear professional or appropriate attire (top to bottom, since you don't know whether or not you're going to stand up on camera), shave or groom as you normally would, and check yourself out before you broadcast to the world. Worried about color? Blue or green are especially good on camera, and choose solid colors: stripes, polka dots, and weird patterns won't decompress well at the other end. You may want to forego jewelry as well: a necklace can clash with a microphone or make noise as you move.
2. Clean up the background. Your camera picks up more than just your face. Make sure what your audience is seeing isn't upsetting (e.g., your messy bed) or confusing ("What is that in the wastebasket? Pantyhose?"). You're best off if you choose a neutral, clean, orderly background to sit or stand in front of when you present. Get the dog or cat or kids out, and keep them out. The focus should be on you, not your surroundings.
3. Control your body language. Eating or chewing gum are out, as are yawning, grimacing, nose or tooth picking, staring out of the window, rolling your eyes, rocking in your chair, scratching, fixing your hair, and anything else that is likely to distract your viewers. Pretend your mom is watching! Don't move too fast or the audience will catch pixellated "stop-jerk" motions that aren't pretty. Sit up straight in your chair: slouching is impolite.
4. Get the right camera angle. We want to see your face, not your bellybutton. Center the frame on your face and upper body, and it's generally best if the camera is a little higher than your eyes: if you're looking up, your double chin disappers and we may not be staring at nose hairs.
5. Adjust the lighting. Natural light is better than flourescent, and lamps are very handy. You want ambient lighting, and you want the lighting to be IN FRONT OF YOU, not coming from behind. Come out of the shadows unless you're trying to scare your audience. Never sit with your back to a window.
6. Look at the camera. This is harder than it sounds. Practice looking at the camera, not at yourself on the computer. Squinting is a no-no (see #3). Of course, you'll have to look away while you multi-task, but don't forget to look directly at your audience often during the session. Staring at the screen without interruption distances you from the audience. A smile doesn't hurt a bit, either: it conveys confidence and reassurance.
7. Control outside sounds if possible. I know: this isn't about video, but audio will affect your looks unless you control it. Open windows will pick up sirens, honking cars, barking dogs, etc.: close them for your session.