When preparing any correspondence or your resume, be as close to perfect as you can.  Here are some suggestions:

• Pay attention to every little detail.  Another way to separate yourself from the crowd.  Minutiae is important!  Correct comma placement, correct spacing, no typos.  Read your letter many times and then have someone else read the letter.  It may be worth the cost to pay someone to confirm the letter has no errors.
• Try not to send e-mails as a shortcut to a formal letter.  E-mails are okay for casual follow-ups but your first one should be done using letter format.  Take the extra time to leave no stone unturned.  Your contact may appreciate the extra effort.
• Spring for good quality, sturdy extra paper.  Use the good stuff for both the resume and the cover letter.  People notice!
• Watch verb tenses and be consistent (generally should be present tense).
• Do not take shortcuts.  Write out entire address, titles are important, do not use contractions.