Here is a PPT I made that introduces 'will' as a modal verb. Then it compares 'can' for ability and 'can' for requesting.
It's really hard to explain the difference between 'can' and 'will'. 'Can' is requesting and 'will' is asking for a favor. There's such a similarity that the only question I could think of that you can't use 'can' is "Will you marry me?"
Then I used a PPT from Mediafire repeating the sentence "Will you help me please?" Then I printed out the sentences from this PPT and cut them up into 8 pieces. The students worked in pairs to put together the sentences, memorize it, and then say it to the teacher. With two teachers we quickly went around the room. The students got one chance to say the sentence correctly. If they did it correctly they win. Then they switched cards and did it again.
2011 UpdateI updated the PPT to clearly show the difference between can and will. 'Will' is less forceful and you can answer "No," (Will you wash the dishes?). Compare that to 'can' which is what your mother would say. You usually don't say 'No,' (Can you wash the dishes?).
Tennis players being asked by fans, "Will you marry me?"
http://www.youtube.com/watch?v=0sLFCwOVrEw