Inform your students about the effects of and consequences of plagiarism at the beginning of the year or semester. While it can seem fairly intuitive that you shouldn't plagiarize, you should ensure that students know this beyond the shadow of a doubt by setting aside a class or two to discuss plagiarism, what it is, and why it is unacceptable.
Tell students to hand in every part of their writing process rather than just the final draft. If you have access to their outline and rough draft, you can get a better idea of how they formed their ideas, and ascertain whether they are plagiarizing or not.
Search key words from a suspected passage in your university library's search engine. While it may be time-consuming to search the entire database, it is possible to narrow your search by subject.
Compensate for minor wording changes by searching for certain key words rather than entire sentences. If you search for "there is a cat," and the student changed the sentence to "a cat is there," the search engine won't find it, even though it is still electronic plagiarism.
Search for suspect passages in search engines on the Internet. This will search through a wealth of academic work, in addition to the variety of "paper mills" and other companies that electronically supply plagiarized work.