There are three elements to a good media project.
1. A Good Story
2. Good Images
3. The Right Music
A good story should tell itself. Don't allow yourself to force parts together that don't fit if the dialog isn't there. Use narration to bridge caps and make connections between thoughts, but only if needed.
Good images are well lit and composed. They tell a story all their own.
The right music is probably the most important of the 3. You can set moods and generate emotions with the right music. You can set the pace of the story with the right rhythm.
If you are not having a custom score created than the most important skill to develope is the ability to hit the beats. Here is a great tutorial on how to do just that.
http://www.studiodaily.com/main/training/howtutorials/10865.html
It is the editors job to weave these parts together and hopefully bring the clients vision to life.
