ETA: Please see the comments below for some helpful hints on how to write formal letters in China and Hong Kong. This post is about letter writing in Taiwan. Are you currently studying Mandarin? Then you might find these posts helpful, especially if you are planning to study at Shida's Mandarin Training Center.
I've noticed on my Sitemeter that I get a lot of hits through "how to write a letter" in Chinese, so after putting it off for a while, I did some intrepid Googling (in Chinese, of course) and found that the following is the general format for formal letters, like those written to a potential business associate or your estranged father: