Teaching Chess

I'm a bit of a chess evangelist and will always be willing to teach new people the rules.  I'm still not sure what the best approach is, or even if I'm any good at teaching.

I'd really welcome any useful tips and advice.  I realise that perhaps the most important thing is making the process interesting to both the learner and the teacher.  (Ever had that bored feeling as the learner randomly pushes pieces round the board?)

Also, how merciful (or not) should you be?

Once a learner starts improving how much tactical advice should you offer?

And one last question, are some people simply unable to do more than learn the moves in chess?