1. Don't use "To Whom It May Concern" as your salutation. If you do that, you might as well just say, "HEY YOU." That's not going to work. Take the time to find out who exactly will be reading the letter. If you have to, call HR and ask.
To the honorable, dashing, irreplaceable Mr. Joseph Phartnog,
Dear Dr. Ranelda Flibbernugget, queen of the social sciences department and future ruler of my heart,
2. Begin with a narrative. Don't start with: "Um, I want the job you posted." That's boring! You're a special individual with a riveting tale to share. Stories capture people's attention, and that's exactly what you're going to do. Share a story that shows 1) why you're qualified for the job and 2) why you want the job.
Last Tuesday at 9:24 a.m., I woke up with a start and skittered down the ladder of the barn in which I currently reside and slipped into the cool air wearing only my boxers. The smell of fresh cow dung assaulted my senses, but I charged toward the street nonetheless. I dove into the road, grabbing a chihuahua as I rolled out of the way of an oncoming semi-truck. As I lay panting in the grass, a small child accompanied by a woman with one blue eye and one green eye approached me. As she thanked me, teary-eyed, for saving her dog, I noticed she was wearing a Nike shirt. That's when I knew I should be the new CEO of your company.
3. Tell the truth, but don't be afraid to brag. This is your time to shine! Did one of your former students say that the lecture you gave as a TA changed his or her life? Note it. Did you boss trust you to run the place while he was gone? Note it.
Before you read my resume, you should know a few important factoids about me. I can play the trumpet through my nose. I have been to Boston in the fall. I once went out on five dates in one day, and that was after turning down three. I have visited P. Sherman 42 Wallaby Way, Sydney. I can do the McDonald's 50 nuggets challenge. I put my pants on two legs at a time. Oh, and I'm Batman.
4. Use the actual wording from the "Requirements" section of the job posting and explain how your experiences meet those requirements. If the application says "must have great attention to detail," write, "Working as the copy editor for college newspaper for two years taught me to pay attention to detail and recognize errors."
You say you want someone with leadership experience? When I was five, I taught my two younger siblings to hide candy under their beds during "nap time" so they'd have something to do while they were supposed to be sleeping. You say you want someone with good people skills? One time my neighbors got some of my mail in their box, and they waited two weeks to give it to me. So the next day, I put a tasmanian devil in their mailbox. Boy, were they surprised.
5. If possible, deliver the letter in person. Unless the instructions explicitly say you must email or mail the application, try delivering it. You may only meet your potential employer's administrative assistant, but that's still better than an impersonal email. Who knows, you might get to shake your future employer's hand.
Busy hiring personnel love surprises. Monday morning at 8 a.m., scale the building from the outside using climbing shoes and nunchucks. If the window isn't open, use a glass cutter to get in. Then find the correct room and play the song "We Will Rock You" on your iPhone. Once it gets to the chorus, pull out your crossbow and send an arrow—with your application attached to it, of course—at your potential employer's desk plant. Bonus points if you don't break it. When he looks up to ask you what you're doing, smile knowingly and reply, "That's for me to know and for you to not know." Then disappear in a cloud of smoke.
If you read this without the words in italics, it's actually good advice. I edit cover letters for people all the time, so if you need help, send them my way. I accept payment in coffee or, you know, money.