Tons of prior threads on the ALO interview. Treat the online aspect the same as any interview. Dress appropriately, even if they only see above the waist. Also, go through the log-in process ahead of time to ensure there are no technical issues with the web connection.
My own favorite pointers:
Dress conservatively and appropriately.
Make eye contact.
Watch your umms and uhhhs. Speak in complete sentences.
Don't blurt an answer. If it is a tough question, it is okay to pause a moment before answering.
Do a Practice interview with a teacher or counselor ahead of time.
Look at the camera not at the screen. DS and I practiced the setup beforehand just to get him used to not looking at the screen. +1 to everything else that AROTC-dad said. Read his links to older posts. Really good stuff in there.
Our son did in person mock interviews yesterday in front of a panel. Intimidating set up, chair in the middle of the room, the panel behind tables and everyone very serious. He said one of them never took eye contact off of him, leaning across the table, felt like his soul was being weighed. Made me laugh.
They gave feedback afterwards. Tip he came home with:
While he answered their questions very well and they were impressed with resume and how he has prepared himself for the academies and how he carried himself, they said he needed to add a little of himself to the answers, in other words share some of himself in them and let them get to know him. Nerves kick in and you may answer perfectly but have to not come across as a robot.
They indicated his resume and answers would certainly make them remember him, but having some personal component as well is very important.