I've also started "pile" work in the back yard. Pile is a misnomer. Several bumpers are placed on the ground a distance away from the dog. The dog is sent repeatedly to fetch from the pile.
Since moving the show into the backyard, his enthusiasm for the trained retrieve has increased. He was bored in the house, but will retrieve with enthusiasm in the backyard.
A point that I have tried to make before is the necessity of truly understanding what motivates your dog. I have a certain "play" routine with the dog that is reserved for outside. No play is available until after trained retrieve work is finished. I think this playtime is what is really motivating the dog to participate. Right now, this specific kind of play is more important that food rewards.
Between each successful fetch instance, I am still rewarding with a treat. But the *big* reward is the play time at the end of fetch work. This is an application of Premack principle. The idea behind Premack is that enjoyable or "highly likely" behaviors can be used to reward less enjoyable or "less likely" rewards. In other words, rewards are actions and behaviors just as much as they are things to eat. Google "Premack Principle" or take a look at this piece I wrote some time ago.
The plan for future work is to increase the distance for a pile of 3 objects. Once he gets that, we'll shorten the distance and increase the number of bumpers. Finally we'll increase the distance to the increased number of bumpers. My intermediate goal is 9 bumpers each at a distance of 50 yards. Only after all that drilling is done, we'll start over and switch to birds.