These days we understand the importance of a sending a cover letter with a CV. It's really hard to find a good job these days, and you'll always want to ensure you make the best of you job application. We lower our chances by not including a cover letter with a CV. Many job applicants are lazy to send a cover letter and assume that the CV would do the talking for them. The CV is a record while a cover letter is a concise introduction and can have more impact on the employer. By including a cover letter, greatly enhance your chances of a successful job application. There are many reasons for sending a CV and cover letter. Following are the important criteria's to make sure your cover letter has:
Make sure not to have a single spelling error or typo. This in turn creates an image of professionalism and competence.
Always be sure you address your cover letter to the person who'll be taking the hiring decisions if possible. Make sure you address the person with all respect you can. Use the appropriate salutations like Mr., Mrs., Professor, Dr., etc. and stuck to professional language Try writing it in your own words. You can use examples from a book is an idea but make it your own. Your cover letter needs to represent you and not someone else.
Read more here: http://www.cv-blog.org/points-to-be-included-in-cover-letter/