Shut Up When You Shoot
When you press the record button, shut up already!
Keep in mind that when the camera is rolling it picks up all the ambient sound, not just what you're focusing on. And you won't be able to separate the unwanted audio out in the editing process.
Don't talk while the camera is rolling, either to yourself or with other members of your team, and no humming.
This is especially important when you're shooting B-roll like natural sound, such as the noise of a busy street or a nature scene, where the sound is critical to the shot.