Avoid: 

• Any disparaging remarks about your current employer or employment situation
• Using cliches such as “I have excellent communication and interpersonal skills, I am a team player”
• Talking about what you want and what you expect.
• Spelling and grammar errors. It does not help you if you call yourself a “Manger” rather than a Manager! Get someone good at English and grammar to proofread your work.
• Never start a letter with “To whom it may concern”. It sounds fake and it is rude! Find out the name of the recruiter, or HR Manager or at the very minimum if you have exhausted all avenues and you still can’t find out a name, use “Dear Hiring Manager” or “Dear Human Resources Manager”

Instead:

• Shape the cover letter around the employer. What do they want? What is important to them? What do they need to know about me that will be of benefit to that company? Match those aspects of your strengths to how you can “take the pain away”. By placing the emphasis on the employer, you take it away from being all about you and help the employer make the right decision.
• Show you have read the advertisement fully. Respond to the requirements in the advertisement with examples of your experience. 
• Replace instances of “your company” with real names such as ABC Corporation; it takes away the generic feel of a letter.