Recently I've been teaching an English-speaking friend some basics about Chinese, but inevitably he got stuck with the tones. I've demonstrated to him the 4 tones with audio files, yet it seems hard for him to distinguish between them. (That being said, he is still able to imitate the pronunciation immediately after listening, though not knowing what tone he is repeating.)
As a native speaker, I have picked up the tones through years of immersion in the language environment, which is of little help to a busy non-native speaker.
What can I do to help my friend perceive the tones? Is there a transition from "cannot perceive" to "can perceive", or are there stages that I can guide him through? How can I describe the differences to him so that he can learn the nuances? Experience from teachers who are used to adult learners (especially learners whose native language is English) would be welcome.