Talk over your shoot with other members of the production team and make sure you’re clear on what shots you need to tell the story. If you’re on your own, write up a list of shots you need.
Write the story beforehand in your head, and list the elements you want to get video of to do that story. Think about what’s going to look good visually, and how your shots are going to come together sequentially.
Shooting documentaries is part technique and part instinct. Depending on the type of documentary you're shooting, you may have to shoot on the fly following the action or everything may already be planned out in advance for example a historical documentary where you're shooting a lot of talking head interviews.