The folowing tips are in no particular order.

The closer the microphone is to the subject the better the sound quality.

Placement of microphones is critical, be aware of surrounding noises, even handling mics generate unwanted noise, wind can play havoc with your recording, always use a wind shield.

Use an external microphone where possible, this will get you much better sound and can be monitored through headphones. The external microphone will also enable to be more flexible in what sounds you pick up.

When Interviewing someone with a microphone it is best to hold it about 6 to 8 inches away from and not in front of the mouth, usually slightly lower than the head. Another important thing is to tell them to ignore the microphone and not to look at it, something most people do.

It can be useful to record ambient or background sounds for later inclusion in your film, especially useful when talking and want some background noise to provide a more interesting film.