When applying for a job within your company, you should write a cover letter in the form of a job proposal to accompany your resume. This will allow you to specifically address why you are the right person for the job and what/how you have contributed to the company. A job proposal will also allow you to show how well you understand the needs of the new position and how you will contribute to the organisation.
Get information. Obtain a copy of the full position description from the human resources department and speak to someone who currently works within the department you are applying to for further insights.
Format your letter just as you would a letter going outside the company. Type your name/contact information in the upper left corner with the date, followed a few lines down by the contact person's name and business address. Include a formal introductory salutation specifically addressed to the hiring manager and a formal closing salutation (such as "Sincerely" or "Regards") at the end of your letter.
Highlight your tenure with the organisation. At the start of the letter/proposal, state how many years you have been with the company and your interest in remaining there. Let them know that your long-term goals include staying with the company and moving up the career ladder.
Make a column with a bulleted list of the skills needed for the new position with a second column next to it matching skill-for-skill with your own experience. This clearly shows that you are qualified for the position.
Mention any awards or recognition that you have received at the organisation. Although you will also include this information on your resume, it is good to type a sentence or two to flush out such important information.
Strictly adhere to the application deadline. Personally turn in your resume and cover letter to the hiring manager.
Don't assume that the hiring manager for this position knows much about you. Use your cover letter/job proposal as a mechanism to express your excitement about the position as well as your qualifications. Don't get too long. Keep your letter to one page with three to four paragraphs total. Don't restate your resume.