Try to be consistent. Make sure you have food for them each time you see them. Crows are so smart that this isn't essential. But it's a general principle in animal training which is, in a sense, what you are doing. Though as you'll discover, it's more like them training you.
Personally, I like to talk to them. I have no idea whether this helps or not. But I treat each one with dignity, greet them when I see them, explain what I'm doing as I'm getting out a treat, ask them how their days have been, that sort of thing.