A well written covering letter can make all the difference to your application. Each covering letter should include a minimum of three reasons why they are suitable for the post. These are the key skills and key achievements that you have that match the role. Make these reasons your most important selling points.
Employers also look for evidence that you want the job, and that you'would fit in with the company and staff. Avoid the corporate-speak formality that makes some covering letters look copied or formulaic, and try to achieve friendly and open feel to the letter.
The covering letter should be no longer than one side of A4 paper and be addressed to an individual rather than to whom it may concern.