How do you train a stray dog?

I feed a nice white germen Shepard stray dog in my neighborhood, and I feed him the following food every day:

1 can of canned dog food (Alpos) on top of 1 full bowl of cheap dry dog food. (Purina)

Keep in mind that many strays are extremely hungry, and that they will gladly eat anything. The canned dog food will fulfill a portion of their daily nutritional value, and the dry dog food will fill their stomach, so that they won't have hunger pains.