Good (or at least readable) student writing starts with a good assignment. Walking into class and announcing that a 10-page research paper is due at the final class won't get you good student writing. Any time you spend preparing a good writing assignment will save you time when you start to respond to the final products.
Start by thinking about your goals for the assignment. What do you want students to learn by doing the task? Will they engage an idea you've raised, explore a connection between the theoretical and practical, report on field research, summarize professional readings on the topic? Or is your goal to determine how much students have learned about the concepts you've presented in lectures? Or perhaps you have one of many other possible goals in mind. Be as specific as you can be about what you want this writing task to do for you and for your students as the first step in your process. (To see more about examining goals as the starting point for designing writing assignments, see http://wac.colostate.edu/aw/teaching/kiefer2000.htm)
Specifically, think in more detail about these issues: