It's well known that recruiters will scan read your cover letter in seconds; so it's vital to give the right "first impression". You need to be clear and concise, and only highlight the key points. Consider including the following:
- Why you are interested in this company?
- Why you are interested in this role?
- Why you? – (why you’re a good match)
- What's your availability (and provide contact details!)
You need to come over as enthusiastic and knowledgeable, so do your research. Make sure you're using the same "language" and style of words as used in the job advert - you don't want it to sound like a "cut and paste" letter - even if it is!
Finally proof read your letter at least twice (leaving a 30 minute gap between reads helps too), and get someone else to read it too.
The whole thing should be no more than a few short paragraphs, and certainly less than a page. If it's more than this, distil it down. The shorter and more to the point it is, the more impact it will have!