So I'll preface this with the disclaimer that I am a canine sports medicine specialist, and the only boarded specialist whose focus is on hunting dogs. My answer is based on what the research has shown and my career experience that has predominantly been with hunting dogs. This isn't meant to say anyone not doing this is wrong, but there is a "best" way, but it isn't the best way for all dogs and I certainly don't get this technical every time with my own girls.
From a performance aspect it has been shown that dog's do best on an empty stomach, this has many factors that go into it, one of which is purely mechanical damage that is done by the food and stool bouncing around while the dog works. Many times performance diarrhea is related to this mechanical damage and not the often blamed stress. This is contrary to how we would feel, but their systems are different than ours and the performance, metabolically is better. Now, there are certain situations that this doesn't work, for example a dog that battles hypoglycemia, I would feed those dogs a small amount in the morning and the majority after the hunt.
Also, there is a recovery window and it is best to feed after work as the body is primed to repair muscle and replenish enzyme systems. Once a dog is calmed down, cooled and recovered is when I typically feed. Again, this can be different...particularly when talking waterfowl dog versus say an upland dog. But to get the best bang I go into the day fasted and feed for timing of recovery.
As far as what to eat, high quality food is important, and this is a can of worms I'd rather not open. I still believe the major manufacturers, generally speaking, produce the best foods in their premium lines. I think too many people justify feeding a garbage food and too many people overpay for high end foods. In the field feeding should be based primarily around simple sugars. I personally use dextrose, but have also used the Purina bar products or the K9 Restart in the water. Using complex foods (protein/fat, etc) actually can be counterproductive as the dog is either performing and so blood flow should be away from the GI tract and towards the muscles and/or the glucose-insulin balance is what we are looking to maintain.
That's my quick and dirty answer in between appointments.
Joe Spoo DVM, CCRT, DACVSMR
Diplomate American College of Veterinary Sports Medicine and Rehabilitation