Letters of application represent who you are to a potential employer, so you should be careful to use them to send a clear message that you are competent, capable, intelligent, and professional. To do this, a letter of application must be grammatically correct, neatly and professionally designed, and clearly written. It must provide a potential employer with information that leads to the conclusion that you are qualified for the job to which you are applying and that you would be a good employee.

For letters of application

always be completely reader-centered. Focus on what you can do for the potential employer rather than the other way around.
use the name of your recipient rather than "Dear Sir or Madam." If you don't know the name, call the company to find out.
it is helpful to state that you are writing to apply for the job, to name the job to which you are applying, and to name the source of the job posting in the first line of your letter.
rather than making general statements that you are qualified for the job and are a hard worker, etc., provide specific information that makes clear that you are qualified and a hard worker. For instance, you might state that you have a B.S. in the required field, extensive coop experience in the area advertised, and that you worked 20 hours a week off-campus while taking 16 hours of classes while you were in school.
it is always safe to use a conservative, readable serif font like Times New Roman or Palatino for letters of application.
never apply for more than one job with one letter.
close with a request for an interview at the convenience of the potential employer.
always include your phone number in your letter even though it is also on your resume.