1) Support the phone or camera to keep it still. Use a tripod, tape the phone to a box, or find another way to avoid the jiggle of hand-held video. This will also prevent the tendency to follow the motion with the camera.
2) Record the event in a well-lit area. Outdoors in daylight is the ideal environment, as even an overcast sky can provide more light than can be found indoors. However, going outside is not always practical. Old overhead projectors can provide a bright light, which has no flicker even when filming in high speed. The brighter the lighting, the less motion blur you will see in the video.
3) Point the camera so that its view is normal to the plane of the motion. Ideally, all the motion would take place at a constant distance from the camera, but that is nearly impossible to achieve. Therefore, arrange the scene to have as little distance variation as possible. This ensures the scale you choose applies to as much of the motion as possible.
4) Place a ruler, meter stick, or other scale item in the same plane as the motion being recorded. Having the scale object at that same distance eliminates parallax error in scaling. Again, the scale must be in the same plane as the motion, not behind or in front of the motion.
5) Remove extraneous people and objects from the video before recording.
6) If you are going to synchronize the video to sensor data, do not rely only on experimental sensor data to accomplish the sync. Instead, include some non-experiment-related event, like a toe tapping a force plate, to accomplish the sync. Sound may also be useful.
If you are using a smartphone for recording videos, note that most default camera software will use a shutter speed that is much too slow for rapid motion such as freefall. Specialized video apps are available that allow the shutter speed to be set. One for iOS is called ProMovie.
Additional information about cameras is available in our Getting Started with Videos Guide at: