Multiple Entry Visa for a American citizen living in Japan

Hello. I am an American Citizen living in Japan. My fiancee is in China now and I would like to apply for a multiple entry 12 month type so I can visit her any time I have a long weekend (about 10 times a year). What would be the best course of action? I've also read that an invitation letter may be necessary. Is there a form for an invitation letter from one person to another person? What else is necessary? Thank you for your help.