I prefer to train with part of the meal (especially in low distraction areas) to cut down on calories. Reward size should be small, just a taste. You never want them to be satisfied.
Once a dog demonstrates that they know the command and are doing it 9 times out of 10, I move to a random rate of reward. It keeps them gambling because they know sometimes they will get the food, sometimes just verbal praise. It should be short random at first... second, fourth, second, third, fifth, third, second and then move to longer and longer in between. I will also choose to reward when the command is done quick and with great enthusiam vs just doing it because they are told.