Research the company. It is essential for you to learn as much as possible about the company and the job. Make sure you have read the job advertisement carefully. Research the corporate website, read the mission statement, and look up employees on LinkedIn. Hiring managers want candidates who know about their company. Try to include a current event, or mention a project that you have the expertise to be of assistance.
Address your letter with care. You may be a perfect fit for the job, but if your salutation is offensive (example “Dear Sirs”) it is less likely that anyone will read it. Try to find the name of the person who will be your boss, and address the letter to them personally (Dear Ms. Smith or Dear Mr. Davidson). If you can’t find this information use a non-offensive generic greeting such as "Dear Hiring Professionals" or "Dear Selection Committee."
Tone matters. The tone of your letter will project your attitude to the reader. Although you can't hear it, the tone in your letter will have the same effect as it has when you speak to someone. If the tone of your letter is cold or unprofessional readers will probably put down the letter. Maintain an upbeat, personable, and professional tone.
Err on the side of formality. Nowadays, cover letters are almost always electronic and are often simply the body of your email to which you attached your resume. Just because you are emailing your cover letter does not mean that it should be any less formal. Print your resume in black ink on regular, white paper. Spell out contractions. Avoid the passive voice. Do not switch verb tenses. Avoid beginning sentences with "there is" or "there are" whenever possible. Do not split infinitives. Spell out acronyms the first time you use them. Refrain from using jargon.
Briefly summarize your career in one to two sentences. Avoid giving the employer a history lesson of your work experience. The letter should be more focused on the future and what you can offer this new employer. Definitely include a few sentences, but be brief with your past.
Tell a story if you can. This part is a little tricky, but the best cover letters lead the reader through an interesting narrative. Explain your experiences in a story-like format that works with the information provided in your resume.
Show (don’t just tell) your reader that you possess the most important skills he seeks. Go in-depth about important experiences/skills and relate them to job requirements. Convince your reader that the company will benefit from hiring you. Illustrate your qualifications with examples.
Include in each paragraph a strong reason why your employer should hire you and how they will benefit from the relationship. This only has to be a sentence, but your letter should make it very clear that the employer would benefit from having you as an employee.
Conclude by saying when and how you’ll get in touch.