Cover letters should set you apart from other candidates by providing a sense of how you think and your eagerness to work for a specific organization. They must clearly point out your skills, knowledge and track record, and explain how these credentials can make a tangible contribution to a prospective employer.

It is worthwhile to demonstrate an understanding of the company's current issues, such as a shift in customer demand or the implications of a recent merger. Most candidates concentrate at their demands (salary, geography, advancement, etc.), not mentioning how their skills, expertise and past accomplishments can benefit the employer. Such cover letters will fail. Job hunting is a form of sales, so you should focus upon the customer.

If you want to have an outstanding cover letter, take into account the following advice:
1. Address your cover letter to a specific person. You can call the company or check its Web site to find out specifically who is likely to be reading your application. Make efforts to learn the contact's name, its correct spelling and his or her title.
2. First of all explain how you’ve learned of the position. Then a recruiter will be able to judge how well you know the firm and position. It may attract additional attention to your person. Tell why you're writing and why it's in the reader's best interest to pay attention.
3. Use your connections (only if you’re sure in good relations between your acquaintance and the recipient). In such case you can start your letter with "Our mutual friend/acquaintance So-in-so thought I should email you". 
4. Stay relevant and brief.  There is no need to reiterate your resume. Mention instead the two or three qualities that distinguish you from other candidates. These qualities should be connected with the job's key requirements. Include description of your most recent accomplishment related to the job you want. If you did any professional development or unpaid work during your period of unemployment, you also may add this information to tour cover letter.
5. Remember your goal. Learn company’s specific needs and explain how you can be useful.
6. It is preferably to write the way you speak. The main purpose of a cover letter is just to inform. So you need to honor the rules of standard English, but avoid stiff, bureaucratic jargon. If you're tempted to use a word in a letter that you wouldn't use in person, replace it with a word that's more conversational.
7. In closing the document, ask for the opportunity to discuss the available opening and indicate when you'll follow up.
8. Don’t allow typos, bad grammar or spelling mistakes damage your cover letter. Make sure you have addressed the letter to the correct person and have included the correct company name and job position.