Middle school students can be tempted to use the first sources they find to get the work done quickly, but they need to slow down to consider the reliability of the source. Encourage students to preview or skim the content to decide if a source is legitimate before selecting it for the project. Tell them to look for evidence of a reliable author, such as the government or an educational institution, and to avoid blogs or other user-generated content.
Try not to confuse students by teaching them MLA for one project and Turabian for another. Most schools have a policy about which citation style all teachers should teach. Be clear about which style your students should use and give them examples for how to cite different types of documents in that citation style. Consider using modeling to demonstrate how the citation style works from the front of the classroom.
Middle school students often do not fully understand what plagiarism is. Discuss the definition for plagiarism and give plenty of examples and scenarios to help students understand the dangers of plagiarism and how to avoid it. Teach students how to paraphrase, write summaries and use quotations in their writing as alternatives to plagiarizing someone else's work. Consider asking students to peer edit one another's projects before turning them in to determine whether or not their words sound like their own.
Don't allow students to decide whether or not they will include a list of sources -- require it. It will be good practice for high school and college courses, and it will emphasize the importance of using and citing references in their work. Most citation styles include specifics for how to create bibliographies, so be consistent with your bibliography instruction. Some middle school students may never have created one, so give them an example and allow them plenty of practice before evaluating one for a grade.