Writing a cover letter is an art form. You may never see one hanging up at an art museum, and you probably aren't going to see a ground-breaking film on the finer points of creating one. But that doesn't mean you don't need a lot of skill to write a great application cover letter.
Part of that skill comes with knowing what to include. It all starts with coming up with a good cover letter opening. A typical opening will start with an introduction of yourself. It may be tempting to open your letter with some elaborate story that describes why working for a nonprofit is one of your lifelong passions, but it will mean nothing if the employer doesn't know who you are. The first paragraph of your cover letter should also specify the job for which you are applying.
The next one or two paragraphs are where you can really shine. This is where you can lay out in specific detail why you are the best fit for the job in question. There aren't many opportunities in life where you are encouraged to brag a little bit, but this is one of them. You should make it a point to include an anecdote. This will help drive homethe points you made, and keep the reader's interest.
The closing of your cover letter should express your interest in talking with the hiring manager (or whoever you are contacting) further. Let them know the best way they can contact you, and the times you are most available to talk. That's really all you need to include in your letter. That's just the basic outline, of course. It's up to you to add little variations to set it apart from the countless others that will be submitted. The most important thing to remember in the whole cover letter writing process is not to go on too long. More than three or four paragraphs and you will risk boring the reader. And that's the last thing you want to do when applying for a job.