How to train students to write high-quality research papers?
edit: To clear a misunderstanding that might have arisen (see the answer of Kimball): the papers described here were not solo-papers. Neither me nor my advisor would ever have the idea to write a paper together and then put only one name on it...
My instructor did a really good job with that (and is still doing it), so I'll explain here approach as I experienced it:
For our first paper, we were sitting in her office, she wrote it and asked my opinion from time to time. I was still really new to everything and agreed most of the time, but I also already learned about some questions that should be considered and, of course, I learned the style she was using first-hand.
For the second paper, we started in the same way. At some point, she told me that I should write a part (the proof of a theorem, a short paragraph,...) myself. Of course we discussed the content first, but I was responsible for writing it down. I already had the first part of the paper that we wrote together to use as a starting point, to adjust my style to it, but I was still ultimately responsible for it myself. Once I finished it, she looked over it, gave me comments and asked me to write a new version. And a new one. And a new one,... until it looked just as good as the rest of the paper she wrote. Of course it would have been easier for her to just write it down herself, but doing it this way I learned how to write myself and I had the success of having something I wrote myself in a paper. (Yes, I had to adjust it a few times, but ultimately I did write the final version that we published)
For the third paper, she told me to write it completely myself. Once again we went through a long process of her commenting on things and me correcting it, but in the end I had a paper that I wrote myself, and I felt really proud of it.
After that, she got me some review jobs, so that I could learn what to look for in a paper, how to recognize strong parts and flaws and (I assume) so that I don't just copy her style but also see other authors and learn from them.
All in all, she did a really good job and with only three publications yet I feel like I am now able to write a paper on my own.
Now let's end with some general disclaimers: The approach given above assumes a motivated student that wants to produce good quality papers and is willing to adjust it until it looks really nice. A lazy student who just wants to increase his paper count with the least effort possible is of course still a problem. Also, your students should be able to produce papers in English. If not, point them towards English courses offered (most likely) at the university, help them install spell-checkers on their computers, etc.
Good writers read a lot. Writing an academic paper in a vacuum can seem overwhelming, but writing an academic paper after studying the presentation and writing style of some well-written papers (preferably in your field; hopefully your field has well-written papers!) gives some idea of what works and what doesn't in an academic piece. In particular for me, my ideas of how to create and present my figures and what makes good discussion and conclusion sections have been entirely influenced by others' work.
ZeroTheHero's suggestion of actively engaging your student in a discussion about why the good papers are good (and the bad papers bad) is a great one.
A nice starting point can be to ask your student to identify 2 good papers and 2 bad papers, and then ask the student to explain (by writing first one-page justification and defending it orally after) why she or he thinks the papers are good or bad, comparing them and explicitly pointing to the differences.