The fundamental rule of audio recording is that the audio source must be "on mic," that is, positioned within the mic’s pickup pattern. The pickup pattern refers to the characteristics of the mic, it depends upon distance, frequency and direction.

3-dimensional illustration Courtesy of Audio Technica 

Click button to select pattern

Signal to Noise ratio refers to the relative strength of the source audio, the desired audio, to the noise, undesired background interference. Noise is the sound(s) associated with a location, it is for this reason Holiwood films are made in Soundstages.

Controlling Signal to Noise begins at the moment sites are being considered for locations.

When evaluating a site it is important to consider its ambient noise. Sometimes what appears to be a noisy site may actually be fine if a time of relative quiet can be found.

Click button to hear selection, check the volume setting on your computer.

For example, if recording in an office, pick a time when normal activity is minimized and also its noises. Turn off any machinery, radios, A/C, telephones, etc. for the duration of the recording and filming session.

Choose your location with audio in mind.

Pick the mic best suited for the situation (location + source).

Whenever possible record each source to its own audio track. 

A primer for microphones - UCSC Electronic Music Studios - in depth information about microphones.

Audio for Video - Part 2 - Video University is a site dedicated to videographers and has a library of articles on video. Check Parts 1 and 3 also.

Educational Materials - SHURE Corp. publishes a number of articles dealing with audio. Some of these are PDF files for downloading.

A Brief Guide to Microphones - Audio Technica's publication about microphones is very informative.

Sergio Damasceno - graduate student in Television, Film and New Media Production.