Once you've written your cover letter, sit down and review it for typos and grammatical inconsistencies. Have someone else proofread it as well. Look for these common errors:
Letters addressed to the correct street address, but to the wrong company or person;
Praising a company for accomplishments that were actually achieved by the company's competitors;
Failing to sign your name;
Failing to include the date;
Misspelling the recruiter's name; and
Misspelling your own name.
Once you're certain that your letter is error free, include it with your résumé and any other materials and send the package to your potential employer.