Although aimed at parent-child communication (and maybe because of it!) P.E.T. (parent effectiveness training, also a long established book/communication system

Although aimed at parent-child communication (and maybe because of it!) P.E.T. (parent effectiveness training, also a long established book/communication system) is quite good in this regard, when there's a great difference in power between parties. For example an additional thing that it stresses is a bigger need for the person with more power, to identify "who owns the problem", before choosing how to communicate about it and also how to help people make their own decisions/find their own solutions.