Keep these things in mind when you write your cover letter:
Avoid cliches. A cliché is a phrase or expression that is so overused that it becomes annoying. Most clichés begin life as clever phrases. The first time around, they convey a distinct message in a funny or striking way, and that makes them repeatable. But after too much use, they become corny and meaníngless.
Don't repeat everything in your CV/resume in your cover letter. A cover letter should complement your cv and explain why you are applying for the job.
Your letter should not cover more than one side of an A4 / a letter size document. The biggest mistake people make when writing a cover letter is making it too long.
Don't send form letters. Each cover letter needs to be tailored to fit the job you are applying for.
Typos and grammatical errors. Run your CV/resume throuhg spellcheck at least once. See "Tips - Résumés"
Should you include a photo? Whether to include a photo in your application depends on where you are. In Sweden, it's considered acceptable, but in the US and some other countries, it is not okay unless you are specifically requested to do so. Research in the US shows that HR managers do not favor including photos and that including one can have an adverse effect on their impression of the candidate.
If you are asked to include a photo on your resume, choose a sensible one with a plain background (preferably a professional headshot). Don't cut your face out of a group photo taken at a social occasion, and keep it professional at all times.