Try to find a location in your home with minimal background visual distractions or clutter.
This can be challenging, as many people use the built-in laptop webcam, but try to position your screen/camera so you are showing head and shoulders at roughly eye-level to your webcam, not a low angle from the neck up with a lot of ceiling. On a related note, no one needs to see how clear your nasal passages are. If you are the dominant subject in the frame, it also cuts down somewhat on background detail because you are blocking most of it.
For a small investment, you can purchase a collapsible laptop stand from Moft. These basic but stylish stands allow for two elevated laptop positions, and collapses so flat you can still pack your laptop - with the stand - in your backback.
If you opt to purchase a separate webcam, you have much more flexibility in positioning the camera, including setting it up on its own small stand. Webcam prices can vary greatly depending on resolution and feature set.
How Do You Sound?
Often, you're using both a microphone AND your laptop speakers in a web conference. This can cause some very excessive feedback in the form of a perpetual echo. If you are running into this situation, consider using a headset or ear buds, so the speakers won't be fighting with the microphone.
What to Wear?
Avoid pinstripes, really bold patterns or boxy, loose outfits. Pinstripes can easily cause a very distracting moiré pattern and loose outfits tend to help add to that extra 10 lbs on the camera. Solid colors are recommended, but you can probably sneak in a subtle pattern for personality. Just watch for that moiré. When in doubt, test with a colleague and get their opinion. More fashion and webcam tips:
Going Live in 3, 2, 1...WAIT!
Before you share your webcam live, check out how things look. Look for distractions or lighting issues. Most web conference software gives you a chance to preview your webcam before going live. If not, the native webcam software or even your OS might offer a utility to help. On the Mac, you can open Photo Booth (possibly for the first time, ever) and use it to preview what your audience will see. The lighting examples at the end of this article were all captured using Photo Booth.
Small LED Ring Light
Usually attached to flexible goose neck, and either USB or AC powered, these lights are quick and easy to set up. Relatively inexpensive, a ring light can provide satisfactory - although somewhat harsh - illumination if the source is close enough. Be aware of the ring light reflection in your glasses.
LED Light Panel (video light)
Brighter than the small ring lights, but also more expensive and larger (size of a large iphone or bigger). A separate stand would be needed to support the light. Better quality light than the ring light.
There are also more specialized products that are more portable and much smaller, such as the LumeCube. But the direct light from this may be harsh and unflattering.
LumeCube is now discounting their video conference kit by $30 USD. Now at $49.95, this is a pretty good deal:
Some examples of basic light and webcam set up are seen below. Note how bright windows can be distracting and how slight changes in camera position can hide distracting background elements. Bright windows can also impact how well lit you appear on camera, as the webcam tries to balance exposure for that bright backlighting.
UPDATE: I've added two examples below, showing how some inexpensive drapes and a curtain rod can have a huge impact to your webcam presence. If what's behind you is busy or cluttered (or you just don't want people peeking into your life), a simple setup with some cheap drapery can be the solution. Note, you will need a light stand or some other vertical support, as well as a clamp to connect the curtain rod to the stand or pole. I've also added a new lighting setup using two tiny LumeCube lights, one bounced into a white poster board for my face lighting, and the other lighting the drape, to add more separation between me and the backdrop.