When creating cover letters, a common mistake is to essentially use the same letter every time. Many applicants change nothing more than the name of the company. This is a bad idea and you can be certain that your generic letter is seen as such by those who read it. The other common mistake is in tone. As a general rule, formal is better. But this is not always the case, though it is a safe fall back position.
I’m going to assume you have done your homework and know enough about the job you are seeking to be specific in your cover letter. If you don’t have that information, then get it before you begin writing. You cannot craft a decent cover letter without specifics.
The tone of the letter will be dictated on the position you are seeking. The rule is that the more authority the position has then the more formal the cover letter should be and the more specific. You want the letter to be brief but complete. It’s important to highlight your skills in relation to the specific job being offered and to explain why you are perfectly suited to the position.
Of course you will address the letter to the person doing the interviewing. If you don’t know who this is, then just call and ask. Address him or her formally throughout your letter. Never shorten someone’s name unless given permission to do so. Save the informal and friendly approach for your friends.
When writing a cover letter you want to get your point across as briefly as possible while showing respect to the reader. The tone should always err on the side of formality.