Place the microphone as close to the actor’s mouth as possible
Having picked the right microphone, the key to recording rich, clean, high-quality location dialogue is to place the microphone as close as possible to the subject. In this way the actor’s voice will be much louder than the background noise. The microphone should also ideally be overhead, pointing downwards at the actor’s mouth. The second best choice is to place the microphone below the bottom frame edge, with the microphone pointing upwards at the actor’s mouth.
The best way to do this is to make the actor(s) get into position, frame them, instruct the boom operator to dip the microphone into the frame, lift it out until it is just out of frame, set the level using a sample line from the actors, and roll!
If you use this technique consistently I guarantee that all you will have to do in post-production to obtain professional, clean-sounding dialogue is set a consistent sound level across the movie, patch up the occasional extraneous noise with clean sound from another take and place the audio cuts wisely.
