Example of a good cover letter (in my opinion):
Dear Chief,
My name is Joe Photog and I'm interested in the videographer position you advertised in B-roll.net
I'm currently a M-F photog at WXXX and have been there for the past three years as where I helped the station convert to P2 cameras and install non-linear editing stations.
I like working as part of a team and my co-workers will confirm I'm a hard working photog with the ability to adapt to a wide range of situations. I can run live trucks and sat trucks, edit tape-to-tape and non-linear and love to fly in the station helicopter. I've had to often work on my own as a field producer and I'd like to bring these skills and abilities with me when I join the staff at WXXX.
Please review my tape and resume and I can be reached at 209-655-5647. I'm willing to relocate and I'm excited at this opportunity to achieve even greater things at your station.
Sincerely,
Joe Photog.
The point, IMO, is to show some skills or abilities not directly mentioned in the resume, in this case, some field producing experience and the eagerness to work as part of a team. Notice I kept it short and sweet and ended it with a statement of "here's what i can do for you!"
Other chiefs may want other things, but this would keep me looking at the resume on the next page.