When you send out your résumé, it is likely to be one of many which will be screened by the recipient. Make sure that yours makes it through the screening process by creating a compelling cover letter.
The letter needs to be concise, yet effective. You want to appear confident, but you don’t want to brag. Demonstrate your written communication skills with an eloquent opening statement as to why you consider yourself to be the candidate for the position. Be careful to use correct punctuation and grammar.
Emphasize your skills and experience as they relate to the position being offered. Demonstrate what value (in skills or qualities) you can bring that will benefit the company. Give specific examples where you have made a difference in your previous position(s) and how those examples are directly relevant to this position. This is especially valuable to help flesh-out a relevant position that is listed on your résumé in less detail.
If you are available at short notice or live close to where you would be working should your application be successful, make that clear.
This is your opportunity to make your application stand-out among the rest, so be sure to make the most of it.