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.