Dear Sir
I am interested in joining the media planning staff at (name of company). The media planning role in an advertising agency is critical to its success, and I have skills and experience that would make me an asset to your company.
I believe that a media planner must be a good researcher and work well under deadlines. In fact, in my summer jobs I have proven that I posses both characteristics. (include examples about how u have succeeded).
I have been selective in identifying advertising agencies for employment but I have been attracted to (name of company) for two reasons. Firstly, I was impressed as how your agency attracted a prestigious account, which had been with a much larger agency. Secondly, the fact that your employees can progress from media to account work fits my career plans exactly.
I am also enclosing my resume, and call you next week to arrange a meeting.
Sincerely