Closer = Better
The closer you are to your microphone, the less you need to compete with surrounding noise. Remember to keep your microphone close by staying about three feet from your on camera mic, or clipping your lapel mic to your breast bone.
External Microphone
If your camera has an external microphone plug, consider using an external microphone for your shoot.
If your camera doesn't have a microphone plug, consider recording a voice over on your computer to lay over your video.
Avoid Common Microphone Mistakes
Don't touch the microphone, or make noise near it (for example: scratching).
Avoid body movements that cause your microphone to shift
Direct your voice toward -- not away from -- your microphone
Listen to Your Audio with Headphones
Headphones are the single most important thing you can do for your audio
They will help isolate what your microphone hears from the ambient noise around you
With Headphones you can identify audio problems when they are still easy to solve