Below are some tips to aid you in preparing a concise cover letter:
Avoid lengthy letters exceeding one page: the letter should be short and snappy. Most cover letters consist of a body of four paragraphs. Limit your paragraphs to four-six lines so you don't overwhelm the reader. Don't just repeat all the information you have on the CV.
Paragraph 1: Introduction of yourself and why you want the job.
Paragraph 2: Relevance of any academic / work experience to the requirements of the particular position; your ability to add value. Show why you fit the job.
Paragraph 3: Why do you want to work for that particular company?
Paragraph 4: What is it about the position that interests you?
Keep in mind the perspective of the person who will be reading your letter. He/she is not interested in what YOU want, but rather in what value you will bring to the organisation. What you convey should prove interesting enough to lead an employer to want to interview you.
Write your letters in the first person; yet vary your sentences with beginning phrases and clauses so that each line does not start with "I". The letter should be well written with use of words like developed / managed / built & organised.