Cover letters should be written in general business letter style and printed on high-quality bond paper that matches your resume paper and envelopes. A one-page cover letter is sufficient for any job. See information below regarding content of individual paragraphs.
Also, please bear in mind that some non-profit organizations have started to ask for longer personal statements (or statements of interest). In this case, the one-page letter may take a different form, as some of the information set out below will need to be included in the personal statement rather than in the cover letter.
If you are sending your application materials in by e-mail, save your cover letter as a PDF file and attach it to your e-mail. This will allow you to retain your formatting when the employer prints out the document.