Writing a cover letter is not that hard if you follow a few simple rules:
Keep your letter short and to the point. Don't write more than one page and more than 4 paragraphs.
Your cover letter should be about two things only – your strengths and how they can benefit the company.
Write one resume and cover letter for every unique job position. That will make your resume match company's wanted candidate as closely as possible.
Your Cover Letter should include only Company Contact Information, Your Personal Contact Information, Letter Greeting, Opening Paragraph, Body Paragraph, Call-To-Action Paragraph and the Letter Closing.
Use a few numbers. In the job market, numbers often speak louder than words. Offer some statistics to illustrate your impact – just as you have done in your targeted resume. Numbers show you understand what company is looking for in an employee – results.