If you are sending your cover letter as an email, write an email subject line that distinguishes you from others. Don't write a vague or generic one such as "Resume" or "Resume, XYZ Inc." Make sure your subject line stands out among all the others by giving it unique wording that includes your name.
For example, let's say there's a job opening at XYZ Inc. for a Website Designer. Three hundred people send emails to the employer, and each applicant attaches a resume. Most of those 300 applicants write a subject line that says either "Resume, Website Designer" or "XYZ Inc. Resume." A subject line like that may be helpful to you, the applicant, because you're trying to keep track of the many emails you're sending various companies. But that subject line does not differentiate your email from the hundreds of others the recruiter got, all of which have the same or similar subject lines.
So get a leg up on your competition by using a unique subject line such as "Grant Miller Resume: Website Designer" that makes it easy for the employer to distinguish your email from the others.