When using a microphone, watch the sound levels of the recording device.
If you’re getting a hum in your sound, replace your cables with shielded cables, or new cables, or at least don’t run them next to or across power cables. Make sure they’re properly grounded.
Before you plan a shoot, record the ambient sound from the location and go somewhere else and really listen to it. Make sure you aren’t missing something that’s going to bedevil you later, such as an elevator, nearby crowd noises, crashing sounds from across the street, whatever. It’s usually easier to find a new location than to clean up crap audio.
Easy on using the Wilhelm Scream as a sound effect. Ten years ago, it was cute, but nowadays enough people know of it that using it actually distracts from your movie and breaks the story.
Get at least one minute of clean ambient sound from every location. This is very important.
Whoever’s holding the microphone boom pole needs headphones. They must be able to hear what the microphone’s picking up.
ADR (depending on who you ask) is Automatic Dialogue Replacement. That’s when your actors come in to the studio and record lines over the outside crappy recording from on set. Pretty much every time you record outside, or in a noisy environment (such as a non-studio), you’re going to have to record ADR. Just plan for it.
If you can’t pipe your sound into your camera, borrow a camera with a microphone jack and use that to record audio. There is no real difference between a ’spensive 16-bit digital recorder and a 10-year old Handicam that records sound in 16 bits. Except, well, cost.