Does anyone know of a good prescriptive mandarin writing style guide
About the usage of punctuations, choice of words, basic principles of writing, my best recommendation is 语文/国文 serial textbooks for primary and middle school students.

On developing an elegant style of writing, there is no simple rules such as "use advanced words", "construct concise sentences", etc. A very important criteria is “文气通达”, which is, roughly speaking, sticking to invariant impact of your writing on the reader, instead of vacillating among styles thus making your writing inconsistent. There is a book I konw, however, discussing the craft of writing, in a more precise way, enriching the knowlege of Chinese language and literature. The title is 《文心》. written by famous educationist 夏丏尊 and 叶圣陶, the book targets on middle school students and teachers, so, it consists of lively stories about two middle schoolboys learning literature, devoid of boredom of listing rules. And the passages the mentions are worth reading as well.(/ω＼)

May it helps.