Always address your cover letter to the appropriate contact at the company. Never start your cover letter with the heading "To Whom It May Concern". It gives an impression that you are not serious about applying for a position and are just one among hundreds of applicants who have applied for the job. Take the time to find out whom you should send your resume to, and direct your letter to the concerned person. This gives you a specific contact with whom you can follow-up, and also creates a positive impact on the potential employer that you were interested enough to do some research on the company.
Don't waste your first paragraph by writing a boring introduction. Use the first paragraph to grab the employer's attention; give the employer the reasons you are qualified for the position. Avoid hackneyed sentences such as "Enclosed please find my resume". The employers can see that your resume is enclosed and they don't need you to tell them. Tell specifically how you learned about the position or the organization - an advertisement in the newspaper, a web site, or a family friend who works at the organization. It is appropriate to mention the name of someone who suggested that you write.
Every cover letter you write should be customised according to the specific company where your are applying for the job. Don't simply change names at the top of the letter. Research the company and address their specific needs. Highlight the benefits that you can offer to the company. The cover letter provides you an opportunity to interpret the top skills on your resume and discuss why they make you the best candidate for the position. Call attention to elements of your background - education, leadership, experience - that are relevant to a position you are seeking. Be as specific as possible, using examples. Reflect your attitude, personality, motivation, enthusiasm, and communication skills. Provide or refer to any information specifically requested in a job advertisement that might not be covered in your resume, such as availability date, or reference to an attached writing sample.
At the end of your letter, make it clear that you are actively pursuing a position with the company and not merely sending your resume for the sake of it. Request an interview, and tell the employer when you will follow up to arrange it.
Don't send a cover letter that contains any typing errors, misspellings, and grammar or punctuation mistakes. It creates a very bad impression. Keep your cover letter brief and concise. Cover letter should not be of more than one page, and it's best to keep it well under a full page. Each paragraph should have no more than one to three sentences. Use simple language and uncomplicated sentence structure. Meticulously eliminate all unnecessary words.