Preventing Plagiarism...How?

I am in my first year teaching high school. I am currently using to prevent my students from plagiarizing, but I am beginning to think that the heavy disadvantages outweigh the easy advantages of saving time and less teacher work.

One of my student's papers was marked as plagiarized and yet she denied the accusation claiming that she was just using sources like I asked. Since the definition of plagiarism includes the intent of a student to pass off another's work as their own, there was no serious action taken against the student.

However, I would certainly like to avoid another situation like this again.

So I turn to my fellow teachers for guidance. What do you do to help your students avoid plagiarism?