Put only three paragraphs on one page: introduction, skills/qualifications, and ASKING for the interview in the last paragraph.
Keep each paragraph to just three or four punchy, well-written sentences. Make it "easy on the eyes."
Allow your text to breathe by including plenty of white space. This means big margins, and double spacing between paragraphs, and one and a half spaces between lines.
Create bullets and numbered lists to help readers scan quickly.
Use bolding occasionally to emphasize important points, and to increase readability. Don't use too much––just a few important lines throughout to break things up a little.
Proofread your cover letter and correct spelling, grammar, and punctuation. This is an obvious one, but if it's not handled, it can ruin any chance you have of landing the interview.
Double check your contact information, and don't forget to hand sign the letter. Nothing is more embarrassing (and deal-breaking) than getting your own phone number or address wrong.