You should always include a cover letter even when applying online or via email.
Many of the same tips for polishing your resume apply to cover letters as well. Read them
A cover letter needs to be confident without being arrogant.
Be brief and stay focused on what an employer asked for in the job posting. Never write more than one page.
Keep sentences short and to the point.
If you are mailing your cover letter use the same paper as your resume. Use resume bond paper in white, cream, or ivory color.
Make sure there are no grammatical errors in your letter. None. Get it checked by Career Services
Always mail a cover letter with your resume. Even if the posting says “send resume to…” include a cover letter. If you are applying online, still send a cover letter.
Your Cover Letter must convey enthusiasm and commitment.
When closing your letter, request an interview and state how you will follow up (phone call, email) and when you will do so.
Whenever possible, address the cover letter to a specific person by name and title.
Sign the letter in blue ink. It implies the letter is original. If not blue then use black. Never use any other color on the cover letter.
Reflect your individuality, but avoid appearing too familiar, overbearing, humorous, or cute.
Focus on what you can do for the company, not what you hope to accomplish for yourself.
Show the employer that you've researched them and the position they posted. Be sincere in your praise, but don't overdo it!
Use simple, clear sentences. Don't try to impress the reader with unusual vocabulary or complicated sentence structures.
Addressing letters, "Dear Sir:" or "Dear Sirs:" is not appropriate as many readers today are women.
Don’t be overly creative with fonts and graphics. Business letters need to look conservative, not like a greeting card.
Don’t spoil the professional look by sending cover letters out in a handwritten envelope. Remember that much corporate mail gets sent with envelope attached. Make sure your envelope matches the professional look of your letter and resume.
Write each cover letter separately, even if you use reuse some of the text each time. Customize each letter with a few sentences that are specific to this job and this company.
Keep copies of everything you send, and follow up as you said you would.