Never send your resume alone! Sending a cover letter is essential to get the employer’s attention. Why should they hire you over someone else? Do you really want the job, or did you just send out 25 resumes here and there?
The cover letter needs to get the employer’s attention quickly. It must be well structured, have a professional look and effectively highlight any relevant experience. In average, the employer will only look at your letter for about 15 seconds, so you have to make sure to get noticed.
To have a professional and uniform look, you can copy the header of your resume. Below the header, address the letter to whom it will be sent.
If you know someone in the company who could influence the recruiter, you can mention him/her in the first paragraph, and then follow with who you are, the job you are seeking, and why you are the best candidate for the job. Why would the company benefit by having you?
Then you can highlight your qualifications, without simply repeating what is on your resume. Describe how the experience you’ve acquired qualifies you for the job. If there was anything in your resume that you wanted to elaborate on, do it here.
End your letter professionally. Make your final pitch, and try to personalise it by showing some knowledge about the company. You can also mention that you hope to hear from them. Keep the closing formal, and do not forget to sign the letter, right above your name.