Write. Review. Target. Revise.
Cover letter writing is an art. While there are suggested formats and samples to review, letters can vary widely. Perhaps the most important thing to remember is that the job of the cover letter is to get you an interview. It is not a repetition of your resume, though it should entice the reader to read your resume.
The actual writing of the cover letter comes after researching the employer, reviewing and deconstructing the job description, and plugging yourself into the job description, i.e. selecting top areas of the job description where you have solid knowledge, skills and/or experience.
Your goal should be to make the reader's task of evaluating you vis a vis the job description easy. Remember that there may be various readers—an assistant with a checklist of qualifications, committee members who wrote the job description, or sometimes a computer scanner that counts key words.
The below samples and outline will get you started or help you refine the letters you are already writing.
Online Writing Resource
The Purdue Online Writing Lab, OWL