This assignment was born when I had one particular class that was struggling with writing. I could tell that a good bit of the problem was their inability/unwillingness to plan well. A veteran teacher gave me this idea and I developed it, ran with it, and loved it. I ended up using it with several classes over several years.
Here's the gist: You give them the topics and detailed instructions in advance. You tell them to prepare on their own, at home, but all the writing will happen in class. The real beauty of it is that they won't know WHICH topic they'll be assigned until writing day...so they have to prepare for ALL the topics. Not only did it nudge many of them to finally see the value of good pre-writing, it also served as a great preparation for the end-of-unit test!
This three-page document also includes a one-page "quick reference" on how to use a quotation in a literary paper. We kept that page in our notebooks for future reference.