Read any prompts thoroughly. Some university applications will spell out what exactly they want you to write about; others will leave the topic open-ended and vague. Carefully assess what the university is asking you to write, before you start the actual writing process. The university's direction should supersede all others.
Make an outline for the content of your letter; then write. Where possible, be specific. For example, if asked why you are a good candidate, discuss (but do not list) job experience you have related to your field of interest, important awards you have won, individual classes or seminars you have taken, etc.
Remember your audience while writing. A university's admissions committee is usually comprised of knowledgeable staff and faculty who must read through a lot of letters. To help your letter of intent stand out, try to maintain a confident, engaging tone and avoid using obvious or superfluous statements, such as "I like this program."
Edit your paper. When you have finished the first draft, set it aside for a while before coming back to it. This will help you notice any errors. Read your letter of intent aloud to identify any awkward phrases or sentences. Repeat this procedure several times until you are satisfied with the result.
Ask several people to proof-read the letter. Ask them what they think you are trying to convey. Preferably, these should be individuals who can be both neutral and honest when critiquing your work. This will give you alternative perspectives and can reveal any problems you missed.
Review your letter of intent one last time. If everything looks good, you are done.