Ask if there is an assistant you can practice with the day before. Ask about your speaking volume and the lighting in the room at that time. If possible, set up your video display to show both your own picture (picture in picture) and who you are speaking to.
Check your hair / makeup / appearance before starting.
Choose your colors carefully. Avoid patterns (like pinstripes), which can create some unfortunate optical illusions at times. Generally speaking darker colors normally translate better (navy blue, royal purple, black, etc.).
Don't wear anything that will clank or jingle - so watch out for loose jewelry, bracelets, dangling earrings, etc. Don't have coins in your pockets. Don't fidget, but don't worry about natural movements. Don't feel like you have to sit there like a statue.
Have a glass of water nearby. Do NOT have any food or gum nearby.
Smile (though not like a loon, as said).
Remember to look at the camera when speaking, not the monitor, so that you will be looking at the interviewer instead of straight ahead when he sees you! (Hopefully they are right on top of each other so it wouldn't matter).
If you find yourself having a verbal stumble, practice pausing. It is far less noticeable to have a short deliberate pause than to hear yourself saying "ummmmm..." for extended periods of time. If necessary to gather your thoughts before answering a question, take a deliberate sip of water to extend your thinking time before answering.
Consider posting key thoughts for yourself (just brief bullet points in very large type) on the wall behind (and ABOVE the video camera) to refresh your memory - things like CONFIDENCE, SMILE, etc. or projects - Teamwork - Showcase Committee, Weakness - xyz, etc.
Most of all, just relax. It's no big deal and it's great practice.