The debate on trigger warnings seems to have run its course for now, with many thoughtful (if you agreed) or outrageous (if you disagreed) contributions. While I agree with many of the thoughtful contributions (see, for example, Ari Kohen, Tressie McMillan Cottom, and David Perry), I'm not too happy about the overall frame of the debate, it's focus on warning students:
Some of the (thoughtful) debate contributors have pointed to the learning hurdles encountered by students with invisible disabilities such as PTSD and the fact that we have to help them overcome these hurdles. While trigger warnings might prevent the encounter with unexpected triggers, the triggers constitute only a portion of these hurdles. Another thoughtful point in the debate is that we should not exclude students who experience discomfort and stress with difficult and conflictual course topics, even if this discomfort does not rise to the level of trauma or other psychological distress. But trigger warnings can very easily exclude those students ("You have been warned: Take this course only if you are worthy of hard questions!"). As a result, trigger warnings are only a partial solution to one problem and make another problem worse.
Instead of adding a trigger warning to the lengthy (thanks, accreditors!) list of rules, instructions, penalties, grading scales etc. that we have to include on our syllabi, why not write a syllabus that helps create an open and inviting course climate? Instead of warning students off, why not invite those who fear conflictual or difficult course material to talk to you about ways to handle the difficult material and succeed in the course? Such a conversation, hopefully at the beginning of the course, is more likely than a trigger warning to encourage students to face the questions that they find threatening and to work with the instructor to do so successfully. In addition, instructors can encourage students to contact disability services and possibly get an access plan if they suspect that their fears are connected to a disability that should be accommodated.
Making your class inviting opens a number of boxes, particularly if you think in terms of inclusive learning, so here are just a few pointers to things that can be done fairly easily. Harnish et al., in an article on Creating the Foundation for a Warm Classroom Climate, focus largely on the type of language instructors use in their syllabi and how they frame the rationale for assignments and explain their own interest in the course topic. They provide a useful list of things that instructors can check when they create their syllabi.
Related to this, with a stronger emphasis on the disciplinary focus of the course, is Ken Bain's Promising Syllabus model, which he summarizes in a short paper (pdf) as well as in his book, What the Best College Teachers Do. Bain's syllabus model encourages instructor and students to enter into a conversation about what students will learn, why they should do so, and how they can document success. If this is successful, the exchange between instructor and student should include a conversation about possible affective and psychological hurdles that the students face.
Among the suggestions most applicable to trigger-warning style situations is one by Christina Petersen, David Langley, and Cheryl Neudauer, in a presentation at the 2013 PODNetwork conference. Petersen and her colleagues propose the inclusion of a paragraph that explicitly invites students to talk to the instructor when they feel overwhelmed and promises that the instructor will try to find a solution with them that helps them succeed. The presentation material used by Petersen and her colleagues, which includes example text for such a paragraph, can be found at the wikiPODia website.