Unless otherwise stated, cover letters aren't a mandatory part of an application package. But if you want to make a favorable impression, demonstrate that you're aware of professional courtesies and give the reader a chance to learn something about you before reading your resume or getting a copy of your personnel file, write a cover letter, even when you're applying for an internal promotion.
A cover letter is a professional courtesy, a greeting of sorts. Its purpose is to introduce who you are, state the position you want and list the materials you're including in your application packet. If you were personally referred by someone, your cover letter should give the contact's name. Typically, a cover letter overlays just your resume, but your application materials also might include a list of references, writing samples or a copy of your professional license.
Cover letters that begin with out-of-the-box openers and catchy exclamations generally aren't suitable for a professional position. In fact, all job seekers should be careful about starting a cover letter that's anything but simple and straightforward. The exception is when a job posting specifically requires that you approach the cover letter or your introduction from an unorthodox approach. For professional promotions, especially with your current employer, stick to language that conveys a serious tone.
The first paragraph of your cover letter for a promotion should clearly identify you as an internal candidate. That means your introduction should include your current position and department or business unit. In addition, indicate how long you have worked for the company and say something positive about your tenure with the company. For example, you could say, "For the past six years, I have thoroughly enjoyed serving as the Sr. Finance Manager for ABC Company and have demonstrated my commitment to the firm's business principles and philosophy. Now, I believe that it's time to expand my role and contribute more to ABC, which is why I'm submitting my qualifications for the Finance Director position (Job Requisition no. 1010)."
You can't go wrong by using standard business letter format for your cover letter. After all, you're seeking a professional role and being mindful of business communications lets the reader know that you take your candidacy for the job seriously and that you want to make a favorable impression. If you're applying for a job with your current employer – and you are, if you're looking for a promotion – don't be informal, regardless of close working relationships you have with your boss or the hiring manager.
Start with a formal greeting in the opening salutation, such as "Dear Mr. Doe," or "Dear Ms. Smith" instead of the reader's first name, even if you are closely acquainted and use the person's first name all the time. Standard business letter format says to type the person's name the formal way, and on the hard copy letter, make a diagonal strike through the formal address and handwrite the person's first name.