Imagine that you are the hiring manager for the company at which you are applying. You might receive a minimum of 10┰ resumes and cover letters on the first few days of posting the job. Then, on the next few days if you happen to see a follow_up cover letter from an applicant saying how impressed they are with the company and would absolutely love an interview, you probably are going to take note and look at their credentials even further. Now let's go back to setting up your follow up cover letter format: First, place a small opening paragraph at the beginning of the page expressing that you are "following up," and that you are still interested in gaining an interview. That's it! Utilizing a second cover letter will literally double your chances at gaining the interview.
"Yours sincerely," should be used when the name of the recipient is known (an introduction os "Dear Ms. Smith:").