"Keep your cover letter brief. Never send a letter that is more than a page in length; half a page is ideal. Be sure to use clear, professional language while steering away from buzzwords, acronyms, jargon, or anything overly personal. Remember, the overall visual impression of your cover letter can be just as important as the written message. Make sure to use crisp, quality stationery. Stick with one font and avoid solid walls of text that make the reader’s eyes bounce right off the page."
And I highly recommend reading "What Employers Want in a Cover Letter" here.