Over the years, Dan and I have standardized the writing assignments to the point where we can have the students purchase a workbook with all the assignments. The workbook gets some revision each time the course is taught, but the assignments have not changed much in the last four years. The workbook is now about 84 700-word pages long, though it has been as long as 128 pages.
This year, we experimented with delivering the workbook electronically, instead of on paper. This encouraged the students to learn to use xdvi and dvi2ps early in the quarter, but it seemed that more of the students were attempting to do the assignments without reading them-making precisely the mistakes we warned about in the assignment. I'll try electronic delivery again, but warn the students earlier about the need to read the assignments.
Despite reusing the same assignments every year, we have had almost no problems with students plagiarizing, as we require first drafts as well as final papers, and concentrate on the rewriting process in class.