I keep the same Food, that I found works well...
Then I buy a Bag of something else that is also top grade, and mix that in. I switch this second bag to diffferent ones from time to time, so the dogs are not eating the same stuff over and over.
IE If I am to give each dog 3 cups of food a day I give 2 of the main food and 1 of the secondary food.
Here is a VERY good link for Dog Food
http://www.dogfoodadvisor.com/dog-food-reviews/dry/
http://www.dogfoodadvisor.com/dog-food-reviews/dry/