Let's start by saying that if you're shooting a live event, you will most likely not get repeat shots, so you have to be prepared A-Z. Let's detail some of the things you must anticipate when filming events.
Get there early enough to get a feel for venue space and coordinate.
Getting there with enough prep time lets you set up your audio and cameras plus it enables you to organize the shots needed with the organizer.
You must have a secure source for audio.
Having a clean feed of audio is critical and probably the first thing we set up upon arriving at the venue. If there is a mix-board, we coordinate with the audio engineers and tap into their feed this is by far the best option as you get a line with a mix of all the microphones that are running through the mixer.
In some situations, we do not have the luxury of a mixer and must run the audio ourselves meaning we set up a field recorder and wire each speaker with lavalier microphones we run those mics through the recorder and sync in post. In the situation of having only two speakers, we can run the mics straight into the camera if we're working with a camera that has good pre-amps.
Getting the best camera position possible is our next task.
We find a position that does not interfere with the attendees while still giving us a good camera angle. If the stage is adequately lit, we can use a zoom lens and set our camera at a considerable distance. If this is a multicam shoot, we coordinate the angles that serve the best dynamics come edit time and separate our cameras and create variety in our focal lengths.
The last Piece of advice
Have a clear understanding of what the event organizers want out of the video. The whole point is for the video to serve there needs so if your not clear on what their looking for you may be making the wrong video.