Include written English communication as well as verbal instruction. In other words, teach students how to write memos and other types of letters they may find themselves having to write in their occupation. It's embarrassing enough to make a mistake on any kind of business letter in one's own language, and students will appreciate the opportunity to ensure their own English skills will prevent them from making a mistake in another language. Focus on one element at a time and give concrete examples of each, from proper salutations to full-blown reports, keeping in mind that what native English speakers take for granted may not easily be understood by others.
Expand your topics to include something every student can relate to. Many students appreciate the opportunity to discuss topics that cover business issues without directly involving their industry. Consider elaborating on the cultural differences between Japanese salarymen and businessmen of English-speaking countries; chances are, your students were given a quick overview of this by their respective companies without being taught the finer points or why certain differences exist.
Structure your lessons to involve activities that rely more on personal, one-on-one discussions than reading out of textbooks and exercises. Japanese schools rely on the rote memorization method when teaching English instead of conversational skills, so while salarymen probably know several English phrases from childhood, they don't always know how to put those phrases to use or sound natural when constructing English phrases. Your students are most likely adults, many of whom have families and can't afford to spend time outside of class doing a lot of written exercises.