Know how the system works. If necessary arrange to have some training. Try to have a simple ‘how to operate’ sheet in the video conference room (perhaps based on the ones on this site), this will be particularly useful if your planned ‘expert’ user is late or fails to attend.
If linking together more than two sites, remember to book a bridge.
Know the dial up numbers. The most common problem with video conferencing is not knowing who is dialling who and not knowing the dial up numbers.
If you are planning on doing anything you have not done before, e.g. sending a presentation from a laptop, using a DVD, viewing PACS images, etc., arrange to test it before the meeting.