This is a Fenzi mantra, actually, from Denise, to Hannah Brannigan to Sue Ailsby to the PhDs that are also trainers: You need to give the dog a choice.
Of course, people who are not +R trainers think that means give the dog a choice to run amok or work but that is not correct. Obviously, you do not let the dog self-reward. There is an element of control, at least of the environment. Hannah wrote a good blog about this after camp last month. You might find it interesting: Camp Lethimmakeachoice It doesn't apply to your situation 100% but there are some good points.