First you have to get good audio. This means not using the on camera microphone on your new camcorder. If you want good audio you have to use a separate microphone to capture the audio. Lavaliere (or lavs) are small microphones that are attached close to the speaker so that they pick up the dialog with out the distracting background noise. Usually these are tucked beneath a collar or under a shirt and usually pick up pretty good audio as long as there isn’t a lot of movement going on. These mics need to be pretty small in order to be hidden (or at least less obvious). The bad part about making a small microphone is that they usually don’t produce the best audio. You always miss something with a lav. The alternative is the shotgun mic. This is a microphone that is super directional (in that it only picks up audio in one direction). The microphone is usually mounted on a long pole (boom or fish pole). The boom is either attached to a stand to keep the microphone near the actor, but just out of the shot, or it is held by an operator who follows the action keeping the microphone pointed at the person who is talking. If there is a lot of moving this can be a very difficult job. One of the hardest parts is holding this pole above your head for a long day of work. You really have to be in shape to be a boom operator.

Even with the best of crew and equipment you sometimes need to re-record some of your dialog after you are done with your primary shoot. Creative Cow has a good article on ADR, the process of replacing the live recorded audio with new audio recorded later in a studio.

The dialog is only one part of the equation. You also need good sound effects and music to set the mood of the movie. The best filmmakers always think about this through the whole process of production. Here is a good article by Randy Thom on the art of sound design.

Here is a clip of Walter Murch, one of the pioneers of sound design, talking about making your sound seem like it is coming from the real world. He talks about getting sound in a location to get the echo of the surroundings. This is something that can be done easily with audio effects now a days, but there is something interesting about the old school methods.

And just for the hell of it here is a clip of Jim Jarmusch ranting about the bad music in movies.