You should slightly personalize your cover letter every time you send it out. Change the name of the receiver, school, city, country etc. Even if it's a small touch, it shows that you have put in effort and are serious about wanting to work for the school.
It can be time consuming personalizing the cover letter if you are sending out 50 emails, but at least personalize your cover letter for the positions you are really interested in. The most important thing to remember is not to send out the wrong personalized cover letter! It's very easy to accidentally send a cover letter expressing your interested for South America to school in Asia! If you are going to customize it, be sure to double check it before you send the email!
Check out the great and easy to follow article on how to write a cover letter. Keep to this style as much as possible.
Have a better template for cover letters? We'd love to hear from you in the comments.