Some Things to remember are: Formality, Spelling, Grammar, Punctuation, Answering all possible questions before they finish reading the letter & Multi-format friendly so you can post if necessary but also email.
Below I have quoted my Cover letter, It was used as an example to show other peers in my group:
I am contacting you as regards to potential work experience within ‘design company name’, from the week commencing dd/mm/yy. Ideally I’m looking for around a two week placement, but I’m flexible as I appreciate you’re a business and will more than likely have a busy schedule.
To give you a little more information about me, my Name is Daniel Hodson and I’m currently studying an Fda In Visual Communication at the Arts Institute, Bournemouth.
I have enclosed a copy of my portfolio, which conveniently is in a handy .pdf format.
Should you need to contact me at a later date my contact deals are the following:
I look forward and anticipate hearing from you.
I think this is of suitable quality to use as a final.