Include your return address (use the same header as your résumé), the current date, the employer's address, and a salutation
If you don't know the employer address, look it up. If you don't have a contact person, get in touch with the employer to determine to whom you should address the letter.
Use business-style format. Follow your salutation with a colon, not a comma. Your text should be aligned to the left.
Introductory Paragraph
Explain who you are and why you're writing (for example, responding to an advertised opening)
Mention the specific job title for which you are applying
Demonstrate knowledge of and/or interest in the employer (for example, a product or service)
Mention your organizational contact/referral, if you have one
Body Paragraph(s)
Tell them why you're a good match for them (how your skills/experiences match their needs--your thesis!)
Mention how your skills, education, and experience are relevent to the position and how they coincide with the employer's needs (provide 2-3 examples from you key selling point list to support your thesis!)
Highlight your strengths and qualifications without repeating everything on your résumé
Use keywords from the job description to tie in with your experience
Illustrate brevity through language that is direct and to the point (not fluffy)
Closing Paragraph
Mention your résumé
State what you want--an interview and/or an opportunity to discuss further your qualifications and employment opportunities (key takeaway: ask for an interview!)
State your plan to follow up, but be careful not to use language that assumes anything (i.e. "I look forward to serving as you summer intern.")
Be assertive ("I will contact you next week to inquire about scheduling an interview.") rather than passive ("I look forward to hearing from you.")
Indicate flexibility as to time and place, or if applying to an out-of-town employer, indicate your willingness to travel for an interview or when you plan to be in the area next, if you will be
Provide your phone number and email address
Express your thanks and appreciation for the employer's time and consideration
