Jen Wells, an equity partner in Team Builder Search, says a good cover letter will not help or harm your chances of landing an interview. A great cover letter will because most if not all employers want to hire people who can write.
If you need editing tips, then listen to this Grammar Girl podcast.
While these 10 suggestions focus on writing a paper for a college course, the suggesions are applicapble to a business paper. Also, these 10 tips center on law school papers but are useful for business writing. Related to these documents, these writing tips cover a variety of business writing topics.
Tim David's post gives suggestions on how to develop a better elevator pitch, which is a presentation that lasts two to four minutes.