I do a lot of recruiting, and see plenty of CL's. It largely depends on what kind of industry you work in, but a few general points here...
1. Be as straight forward and concise with your intent. Honesty and humor are essential as well, but don't make the whole thing a joke. (unless you are applying as a comedic writer...)
2. Figure out to whom you need to address the application. Starting "Dear Hiring manager" or "to whom it may concern" won't cut it. Call up, ask around, do your research.
3. Do NOT repeat point for point what is in your CV/resume. Redundancy is irritating, and wastes time. Highlight one or two key points, but do so in anecdotal form. If your resume says "improved sales 40% between June + October," talk about how that happened in the cover letter. What steps did you take? With whom did you work to achieve it? How did it directly benefit your company at the time? Why were you critical in making it happen, as opposed to any other schmo in your office/workplace? You don't want a novel, but you want to put a small story to a bullet point or credential. This will help personalize your application.
4. Check your formatting, spelling, and grammar. After you have it all correct, give it to at least 2 other people to check. If possible, have someone who doesn't know the position for which you are applying read it, then ask them what they think you are going for. If they cannot clearly see what the job is and why you are qualified, your letter is not doing its job.
I'm sure I'll think of some more later. I'll try to put together a quick list of some of my favorite 1 or 2 liners I've seen...
