Plan ahead and start early. Have a focused message. Keep the audience in mind. Think about what you need to record, where you need to record it, and the types of footage you need to get.
At a minimum, create a shot list. Keep a list of the types of shots and moments you want to get, and how you want to get them. You'll save countless hours if you do (and get the exact shots you'll need, instead of having to go back after the fact and try to recreate particular scenes).
You can also try creating a storyboard of your project, which is like a graphic outline, with text and/or images.
If you're creating a dramatic storyline with actors and props, write the first draft of your script. Otherwise, going in without one is like trying to navigate a boat without a clear sense of where you're going.
Likewise, if you're conducting an interview, jot down a list of questions to ask. You'll feel more prepared if you do, and catch yourself if your mind goes blank (sometimes people get interview fright too!).