A well-written cover letter should allow for a natural and logical progression. 

In the first paragraph, introduce yourself briefly, mention the position you are applying to, and specify how you learned about the opening.  This step is important because it will provide context for the rest of the letter and make you more personable.

In the following paragraph(s), mention why you are interested in the opening and discuss how your background makes you qualified for the position. 

You should conclude the cover letter with a request for an interview.  Also make sure to state your contact information (phone and e-mail) and to thank the recruiter for his or her time and consideration.