Avoid brightly-colored paper and ink at all costs. If you are gainfully employed, do not use your company stationery, no matter how classy it is. That should be self-evident. As stated above, do your homework. Research each market and send them only what they ask to see.
Chances are, they'll spare themselves the trouble of finding out.
Proofread your letter carefully for errors, then show it to some other trustworthy soul. Our own mistakes are often the hardest to spot, since we know what's supposed to be there on the page and tend to see it whether it's really there or not. Finally, retype or print out a crisp, blameless copy of the corrected letter. While the purpose of this article is to help you make that wonderful first novel sale, I'd like to finish up with a few thoughts on rejections. Fear of rejection is a reality for most of us.