What research shows how assessment design can minimize plagiarism?
According to your profile, you teach management. So we are looking at writing, rather than equations?  

This review article discusses steps taken at a university in Australia, and provides some lit review. It is open-source. In regards to research on assessment design, it recommends:


Regularly change the questions/prompts 
Projects should require original analysis rather than be summaries or definitions.
More marks should be associated with the process of project creation, rather than the final draft. Examples of intermediate assignments are research journals, annotated bibliographies, and first drafts.
Change product from text-only to something else - a poster, an oral presentation.


Above is the "answer" to your question. You may also want to consider why your plagiarism is suddenly increasing. The linked article points out that many students plagiarize because of lack of training in how to effectively write without copying a source. If you have a cohort of students from a background of poorer preparation, I would recommend a module on how to write without plagiarism. 

Best of luck.