Not everyone you are applying to wants to see a cover letter but when you do send one it is just as important as your resume. It is probably the first piece of communication you are presenting and your first impression must be a good one.
To help you prepare a winning cover letter follow this checklist:
Always address it to a specific person.
Keep it short, particularly if you are emailing it. No more than 1 page, less if preferable.
Tailor the letter to that particular job.
Make an impact in the first paragraph. Let them know immediately why they should hire you.
Show them that you are knowledgeable about the company and the industry. This tells them your application is targeted and you have done your research homework.
Use language that sounds like you, not stiff or pretentious.
Use action words to describe your accomplishments.
Ask for the job interview, meeting, phone conversation or whatever is the next logical step.
If you are mailing your letter make sure the envelope matches the letter (name, title, company and address)
Check, re-check and then check again for spelling and grammar mistakes. Errors of this kind are inexcusable and reflect poorly on you.
Keep copies of all your correspondence