One thing I do with my students when I notice they're struggling is remind them that I'm available to answer any questions for them or help them with study tips either after class, during my office hours, via e-mail, or by appointment. Not only does this assure your students that you're there for them, but it also covers your butt if a student complains that you didn't do anything to help them when you gave them numerous opportunities to get help.
Also scare the bejesus out of them the first day of class about plagiarism and cheating. I don't know if you use Blackboard at all, but there's an option on there to have students submit their assignments via SafeAssign, which checks for plagiarism. Reminding your students of that will (hopefully) tell them that you will know right away if they're cheating. I've been an adjunct for three years and have never run into a plagiarism problem because of this.