State your biggest achievement. This has to be in an area relevant to the one you are trying to break into with this letter. Say a few things about your success and let your potential employer know how you could use this knowledge and expertise when working for him.
Grab your reader's attention by mentioning the name of a colleague who has referred you, or a mutual friend who might have put you and this employer in touch. This makes it almost certain that you will stand out from other candidates.
Address the reader by his last name. If you do not know it, call or look online and find out. Make sure you know who is dealing with the resumes and cover letters of the company to which you are applying. Get that person's attention from the start by addressing him by his name (for example, Dear Mr. Jones) rather than a general salutation (Dear Sir/Madam).
Mention any previous communication or conversation. Carry on with your letter as if you were continuing your conversation from where you left it last time.
Show enthusiasm. Say that you have been waiting for an opportunity like this to emerge and that this seems to be exactly what you were looking for.
Quote a relevant industry article, which proves your experience, research and expertise in the area. It also makes clear that you investigated before contacting future employers and know what is happening in your field.
Make your reasons clear for writing to the person you are addressing. Try not to tire the reader by giving irrelevant information. State why you are contacting him and sending him your resume, where you heard about the job and give him a reason to keep reading.
State your skills. Stress all your positive features that are relevant to the job in question. Let the employer know why you would be an asset to his company. If you are, for example, dependable, organized, quick and efficient or highly motivated, emphasize this quality.
Highlight your experience. This will give you an advantage over other candidates who might be just starting out in the area, or over people applying for the same job who mention educational degrees achieved. State your years of experience and leave your educational track record for the resume.
Start with a reference. If someone in the industry has acclaimed you, make it known from the beginning of your letter. Whether this is a past employer or a competition you have won, quote the person congratulating you and go on to say who spoke these words and why you think they are relevant.