How I would do this...

[Krankenstein]

Obviously they are going to be turning Cena....I think thats a given.  I like how Kane is sort of coaxing him along.  Here is what I would have done with last Monday...in the match with Cena and Ryder:

1) Match isnt over....Ryder is in the ring with Henry...Kane's music hits....suddenly the lights go out...you hear commotion in the ring like a chair being used on someone....lights come on...Ryder is out cold in middle of ring with chair next to Cena...

2) Match is over...again, Kane's music hits...lights go out...you hear commotion in the ring....lights come on...again, Ryder is out cold...Cena is standing next to him.  Kane is then up in the rafters or somewhere far from ring.  Ryder wakes up...thinks Cena is one who put him out.  Doesn't accept Cena's apology or explanation

This happens to multiple 'partners' in tag matches....thus Cena gets tired of people accusing him of being bad...and says fuck it, turns heel

[Montague]

Foley cut some INCREDIBLE promos when he turned against (the original) ECW and its fans towards the end of his tenure there.
It'd be nice if WWE got Mick to coach Cena a bit, but I don't know if that's really something that you can teach or learn.
It seems that the guys who have it always did, but those who don't never really catch on.

[leaky_frog]

  IF they turn him, and I'm still not convinced they're going that way, I'd like to see his character be almost EXACTLY the same on the mic, with the dirty play increasing in the ring.  It's the smiling at everyone who hates him and acknowledging it at the shows that's getting him heat from me, not sure if it's intentional or not.  Let him keep being shiny happy Cena, but taking things over the line in the ring, especially with some likable mid-carders, and draw some heat.  Cena should be watching a lot of Hogan tapes circa '96, because he's now that guy, ready to turn hard.  I hope he does it well if it happens, because even though I'm not a fan of his ring work, he seems like a genuinely good guy, and I'd like to see him continue to succeed.