The difference between written language and conversational language in Japanese is much greater than in English. Japanese letters often use classical grammar patterns which are seldom used in conversation. Although there are no particular rules when writing to close friends, there are many set expressions and honorific expressions (keigo) used in formal letters. A conversational style is not usually used when writing formal letters.
See a list of expressions and their explanations at http://japanese.about.com/od/grammarlessons/a/101401.htm?nl=1