Clarify to what extent collaboration is acceptable.
Have students create the expectations or rubric for the assignment with you.
Require specific formatting.
Use unusual combinations of terminology or texts.
Use class discussion(s) as a source.
Provide source restrictions.
Use opening Write to Learns that ask students to synthesize their knowledge about a source or set of sources are also a possibility.
Require disk copies that can be fed into scanning services on the Internet.
Add oral presentations.
Have students keep a paper or project log that documents and engages with the research process.
Require significantly revised multiple drafts.
Along with the idea of multiple drafts, break the assignment into parts that are to be turned in at different stages of the process. Examples of parts include short Summary/Response papers for each source or a set of sources or an Annotated Bibliography. The Annotated Bibliography can be broken down into sets of sources as well. For example, students might be required to turn in 3-4 sources at the end of the first week of the assignment, 3-4 at the end of the second, etc. As Reade W. Dornan et al. points out, the individual parts do not need to be graded, but they will help hold the student accountable and will also familiarize you with the student's thinking and writing about h/er subject (146). Portfolio grades, or, as Robert Harris suggests, assigning points for each component so that a student without them will not pass the portfolio, are also helpful.
Require photocopies of sources. Students don't need to turn in an entire book; a copy of the page(s) from which they quoted will suffice. Be sure to let students know of this requirement at the start of the assignment so they aren't surprised.