This will gain you the ability to cross many lines you would not be able to as a demonstrator. In these sorts of situations most attorneys find it useful to show the physical relationships between the actors. The most typical charges at demonstrations revolve around things like people crossing a line on the street or standing on the curb instead of the sidewalk. Many an arrest charge has been dismissed simply because it's been shown a person was not standing where the officer said they were. If you begin videotaping an incident, it's a good idea to leave the camera running until the incident is over.
Many prosecutors try to use the gaps in a videotape to attack it.
The most basic technique for good pictures is to try to brace yourself against something so that your camera doesn't shake. Since you're trying to cover a lot of the action, leaving your camera in wide angle is recommended. What is important to videotape depends a lot upon what occurs. Sometimes cops seem to be trying to fill a minimum quota for numbers of arrests. In these cases it's a good idea to be proactive.