A few things to keep in mind: Whenever you're relying on free extras, they will always show up 2 hours late. Tell them to show up at 10, so that by 12 most of them have shown up. Don't put too much pressure on yourself, especially for a music video. Unlike a film where every little thing has to be planned out beforehand, a music video gives you the most freedom to experiment and showcase your creativity. Don't waste energy thinking of the greatest video ever! Just go out there and shoot. Some of my best work was on the fly, guerilla style!
Editing: If you are shooting slow-mo while the artist is singing in front of the camera, make sure the song is played and sung at twice the speed. You'll thank me when you're in post. It'll make your life easier.
Clientele: Start by doing a couple of free vids, get that resume/portfolio going. From my experience, the key to eventually succeeding is persistence and dedication. Stay at it, believe, and there's no doubt in mind you won't make it. You just gotta believe.
The rest is up to you...
Mario