[DaveESL]

I discourage my students from spending a lot of time studying idioms. In my experience, they're often presented out of context (ie a list or worksheet with no other purpose than to introduce idioms). I agree with what Chris says about them being so culture specific, too. I advise my students to wait and see what idioms they encounter naturally (in their reading and listening) and learn them as they go. Each student will probably have a very different experience. I take the same approach with slang.