I receive hundreds of resumes each month, but I only have a select few jobs to fill. I don't have time to interview every applicant although I do look at every resume. And those resumes with cover letters (and selected reference letters) usually go to the top of the stack for consideration.
You will want to personalize your cover letter with the name of the position you are applying for, as well as the company you are applying at. You can create a template for yourself, but make sure each and every one of your cover letters, or email text, is personalized in one way or another. Humorously, I usually receive several resumes each week with cover letters addressing other companies or job posts.
If you are authoring your cover letter as an email, I recommend that you never send an obviously boilerplate “Attached please find résumé”. Do tell a bit about yourself, your interests and skills and a short summary (no bullets) of your career. There are many personal questions an interviewer is not allowed to ask by law, but if voluntarily supplied by you could show that you would be a good fit for a client's work culture and that could get you a first interview. But it works both ways; never list anything in a cover letter that could eliminate you from being considered for the job you want.
Remember that a staffing agency works for a client, and the client is our customer. Our clients go through an extensive interview process which tells us what they are looking for in their next employee. Let your cover letter paint a true picture about yourself. It will help us present you as the best fit candidate.
Always mention mutual contacts, and proofread carefully.
