Here is an article for students on how to talk to their professors from Michael Leddy and another from Dustin Wax. These might make a good appendix for your syllabus and/or great suggestions to share with your advisees.
I love the advice to "Tell the truth...your average professor has pretty much heard it all...If a professor thinks s/he’s being played, they’re not going to respond very well to whatever request you have to make, so you might as well be honest. If you feel you absolutely must lie, at least make it a huge flaming whopper of a lie, so the professor can get a good laugh when they share it at the next faculty meeting."
Thanks to Jeff Hess for pointing me to this.