Because recording room tone is usually done at the end of an interview, it's easy to forget. These essential 30-seconds of "silence" that can be overlooked in one's haste to clean up equipment. I've kicked myself many times.
So here's my solution: I write the word tone on my hand. That way, I'm constantly reminded of the task. Even if I forget to record it right after the interview, I'll see my impromptu tattoo once I start wrapping up cables.