We don’t work on graduate employment collateral any more but this his post from Alison Doyle, an American employment specialist and expert, is very good.
She writes about internship cover letters. It might help some applicants out.
Your cover letter should be tailored to the specific internship and should include examples from your work, academic, and extracurricular experiences.
Use a business format when sending a cover letter. Include your contact information at the top, the date, and the contact information.
Be sure to provide a proper salutation at the bottom. If you are sending a cover letter by email you do not have to include the contact information at the top.
Make sure to write a unique cover letter for each internship you apply for. Highlight skills and abilities that relate to the specific internship. Convince the reader that you will be an asset.
One way to individualise your letter is to use keywords from the internship listing. For example, if the listing says the intern needs to have excellent “time management skills,” include a brief example of how you have demonstrated those.
In the letter, you can mention academic experience, especially if you have limited work experience. For example, if the internship requires you to work as part of a team, provide an example of a successful team project you worked on during one of your school courses.