Cover letters
A cover letter is a document that pretty much all of us have to write. It's that thing that goes with your resume, a letter required by your potential employers. But what is the purpose of this letter? Your resume should tell the employer everything they need to know: where you went school, where you've worked, what responsibilities you've had, how long you've been at a job or in school, what your computer skills are, etc. What else do these people need from you?
The truth is that at this point almost everyone's resume looks the same. We all went to college. Grade inflation means that pretty much everyone has a 3.0 GPA or higher. Most of us have held a job of some kind, and just about everyone is involved in some kind of activity beyond work and school. We figured out what the ideal employee looks like and then we developed a society that produces those people, so everyone is equally employable.
However, even when all candidates for a job have exactly the same skills, there are little nuances to the people, their attitude, their mannerisms, their cultural backgrounds and experiences, the essential characteristics of who they are that differentiate people from each other. These differences are not in evidence on a resume, which simply lists all the things that you have done. What you've done might reveal a hint about who you are--for example, if you have spent more time volunteering than working, people can assume that you are a) not particularly interested in money and b) care a lot about the well-being of other people. Or if you have been the president of every club, organization, sports team (ok, those are captains, I know) and everything else you've ever laid a pinky finger on, then people can assume you are a) charismatic and b) like leading people. But these generalizations only cover the very superficial aspects of who you are.
People like stories. We respond to the narrative of a person and can extrapolate a lot of details from that one thing. Your resume is about information. Your cover letter is the story of who you are as an employee. To work, your cover letter should point to about two times in your entire life when you were the epitome of successful. You want to tell the story, in all its details, of how you solved a customer problem effectively. This does not mean say "A customer had a problem and I solved it effectively." It means tell the reader of your cover letter what happened. "When a customer called indicating that a paper shipment had not arrived at the warehouse and lamenting that printing would be delayed, I called the paper mill to find out if the paper had been shipped, tracked the shipment, discovered that it had been lost in the warehouse, identified another shipment that was on schedule to deliver early, and rerouted that shipment to the customer in need. The customer received the product within the hour and was able to complete the print order on time. Through updating the paperwork and manipulating the inventory, I was able to solve the problem and cover my tracks so that the receiver of the redirected shipment never knew a change had been made."
Keep in mind that the people reading cover letters are just like people reading anything. They want to be entertained; they want to like you; they want to stop reading cover letters and just hire someone already. Make their job easier. Be that person who is likable, entertaining, and so good at their job that they can hire you right then and move on to their next order of business.