I'm trying something along the same lines with Ziggy now. I'm keeping training sessions short and sweet - ending them when he's still enthusiastic and has drive - before he makes the Ziggy slide into ignoring me. Right now that's about 10 minutes - max.
My theory is that if I'm always fun, and I end it when we're still really having fun, then he'll want to come back for more. And maybe someday we can extend the amount of time we play, and we'll slowly build drive.
Just like George, but with dog training. I'll let you know if it works.