In some ways the cover letter is even more important than your résumé. Most 
employers are very busy and will use the cover letter to quickly decide if you 
are suitable, before they decide to read on.

Customise every cover letter
You should customise every cover letter you write so it is relevant to the employer and position.

Focus on what the employer wants to know
Don't focus on what you want from them. Through your cover letter you need to try and convince them that you can do the job, that you’ll do a great job and that you’ll fit in and be an asset to their organisation.

Make sure you include your contact details
Also include the date and the details of the person you are writing to. 

Keep your letter to no more than one page
You don’t want to overwhelm the employer with too much information. Your cover letter will help the employer to decide if they want to read your résumé in detail.

In the opening paragraph, explain the purpose of your letter
Mention the position you are applying for and where you heard about it or where it was advertised. Also try and make a point that will encourage them to keep reading.  Think about which of your selling points will be most appealing to the employer. 

In the next couple of paragraphs, show you can do the job and do it really well
Before starting your letter, clearly identify what skills and experience the employer is looking for (hint: look at the job description, if you have one, and then highlight the qualities that make you the perfect match for the position).  

Next highlight how keen you are to work for the organisation and why.
This shows you have taken the time to find out about them.  Also try and give some information about what type of person you are. Keep in mind what you know about the employer and mention the personal qualities you have that you think they are looking for. 
It is very important to employers that you fit in with their culture and get along with other staff.

Before finishing, thank them for taking the time to review your application
Also mention any attachments including your résumé and any work examples.

Once you have successfully applied for a job, don’t forget one of the most important things, the follow up phone call. To show your willingness to get the job, wait approximately a week after applying for a job before you call. Calling your prospective employer can make a difference between getting the job and getting lost in the hundreds of other applications.