I've incorporated idioms into quite a few of my classes this year. I think they're of utmost importance for ESL learners. If they are to properly understand native English speakers they need to understand idioms, as our regular speech is littered with them.
So far, we've talked about color idioms (out of the blue, green with envy, a white lie) and food idioms (a lemon, a piece of cake, cheesy). I've stressed again and again with my students that they need to incorporate idioms into their speech.
I've also noticed that many Koreans do not have very polished email skills, to put it lightly. So, I'm also working dilligently with my students on improving their email response time, grammar, and format. Their current assignment is to pretend that they are the manager of a small electronics store. They have 12 employees. They need to fire 4 of the employees due to slow sales. So, they need to send an email to their employees calling a meeting. They have to give a specific time and ask their employees to bring a list of their contributions from the last year (ala American Beauty).
After putting so much stress on the importance of idioms, I shouldn't have been surprised when some of my more enthusiatic learners combined the two lessons. My student Mina wrote....."at meeting in Wednesday on 9:00 pm 4 of you will get the boot."
I think I'll address idiom etiquette in my next class.