Don't repeat information already contained in your resume, e.g., "as you can see from my resume, I’ve been in the industry five years.…" Instead, begin with a brief sentence or two indicating what about the company intrigues you and why you’re forwarding a resume. Use keywords from the industry and the job description or ad. Make sure the letter states clearly which job you're interested in and, if applicable, reference the ad to which you're responding.
Next, identify either a) your two best professional traits, or b) the two traits you believe are most pertinent to the job. Come up with an anecdote exemplifying each trait. Address each in a separate paragraph referring to the trait, the anecdote, and why you think the trait pertains to the job. (See the discussion below regarding interview preparation.)
Close the letter by telling them you want to interview and that you'll follow up in a few days. Then follow up!
Avoid sending your cover letter and resume to HR if possible. Do your research, call the company, find out who your boss would be, and send it to that person. You'll probably end up in HR, but don't start out there.