how to learn good Japanese writing style

Almost all Japanese textbooks are geared toward teaching basic hiragana, katakana and kanji, along with some basic grammar. I have been writing the way that I speak, but apparently that doesn't cut it. Japanese writing style is so different from English (for one thing, long sentences are the norm, where long sentences are poor style in English). How does one learn how to write like a native?

I don't think that the way native Japanese speakers/writers write is necessarily "good" (this coming from a Japanese). At times almost a paragraph-long sentence drives me crazy lol. For clarity, I prefer to write in shorter sentences. There isn't really much education done in early school years in terms of "logical" writing or essays for persuading others. We are (this is what I feel looking back at my school classes) taught to convey atmosphere, feelings, and moods, rather than clearly writing "logic," which of course is served better in shorter sentences.

If you want to learn (be it good or bad) how the natives write, I think all you can do is read a lot of those text and learn to grasp the "flow" of the sentences and imitate.

The above is assuming that you are already at a very advanced level in Japanese. If you are at intermediate level or so, I advise against intentionally trying to form longer sentences just for the shape and possibly losing clarity in the effort :) Then it is better to use conjunctions to connect shorter sentences for clearer communication.