When recording audio while filming it is inevitable that you are going to have background noise. The world itself is a collage of natural and man-made sounds that cut through the silence and create a unique mix of audio. Since you would like to cut down on ambient noise as much as possible there are a few things you can do to reduce the intrusion of this background noise.
Twilight of the Machines
No matter where you are at, whether in a studio or some sort of outdoor location, it is best to turn off as many machines as you possibly can. This means turn off fans, appliances, computers, and anything else that creates a consistent hum or sound pattern. These sounds will inevitably show up in your audio and this is a simple thing you can do to get rid of it. If you are indoors make sure that all windows are shut and secured tightly to cut down on the amount of outside noise that gets in.
No Cell Phone Zone
This may seem like it goes along with turning off electrical devices, but it is especially important that everybody near by shuts off their cell phones completely. The main reason for this is that the signal of a cell phone receiving a call interrupts the audio feed and creates a loud static sound in the recording. It is not good enough for people to turn off their ringers, but they need to have the device gone completely. This is not the kind of sound interference that you will ever be able to edit out.
Hide From the Wind
All microphones need to have some type of wind screen on them unless you are in a specially controlled studio where you need particularly sensitive audio recordings. Even the slightest breeze or accidental movement will bring unintended feedback from the microphone so it is important to use some type of cover on them to cut down on this. These peripherals usually do not cut down on the function of the microphone and will save you from unruly conditions.
Turn Off That Loud Music!
Be careful of background sounds with creative patterns, such as music or television. You can not record room tone of these things to edit out or lay over because each second of the sound is different. If you cannot shut these things out make sure to have someone recording the entire audio segment so you can play it in its uninterrupted entirety in the final edited video. If you just get random snippets here and there because of your take selection then it will sound random and the sequence will not flow naturally.
It may seem obvious at this point but you cannot avoid recording room tone. This is a crucial thing that allows you to lay the consistent background sound in the final editing track so that you can fill audio gaps. It is the last defense against background sound, and it may be the best choice you have.