I currently reward with food. We haven't perfected catching the spit food yet.
He has good drive for the toy(better than food), but I have not added that to the exercise very much at all... a few times. I plan to use the toy to build drive for the DB.. I usually tease with the toy, get drive high, throw DB and then reward with toy. Flinks stuff, I think. But, I was trying to get the whole behavior pretty good before I did that.
Previous dogs, I have added compulsion to motivate and clean up the retrieve once the basics were known. This dog is 15 months old. I think I am going slow with him. Not sure why. I just feel he will come right along with compulsion but I wanted to wait a bit for maturing on this exercise.