I do not have picky eaters. I simply put the food down twice/day and give them 10 - 15 minutes to eat. Then, I pick up the bowls. If they don't eat one meal and the food goes away, more than likely they will eat the next. Sometimes it may take a couple of days, but I have yet to have one starve himself.
If you change their food every time they refuse to eat, they know that they can train you to go find a new food.
I take in many pups and never go through the picky stuff that I see so often posted.